Download the Minerals and Rocks

HOW TO KNOW
the Minerals and Rocks
RICHARD M. PEARL
Department of Geotogy
Colorado Cottege, Colorado Springs, Colorlllh
McGRAW-HILL BOOK COMPANY. INC.
New York Toronto London
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HOW TO KNOW
the Minerals and Rocks
Books by Richard M. Pearl
GUIDE TO CEO LOGIC LITERATURE
SUCCESSFUL PROSPECTING AND MINERAL COLLECTING
1001 QUESTIONS ANSWERED ABOUT THE MINERAL KINGDOM
POPULAR GEMOLOGY
ROCKS AND MINERALS
GEOLOGY; PHYSICAL AND HISTORICAL
COLORADO GEM TRAILS AND MINERAL GUIDE
NATURE AS SCULPTOR: A CEOLOGIC INTERPRETATION OF
COLORADO SCENERY
AMERlCA'S MOUNTAIN:
REGION
PIKES PEAK AND THE PIKES PEAK
MINERAL COLLECTORS HANDBOOK
COLORADO GEM TRAILS
HOW TO KNOW THE MINERALS AND ROCKS
With Dr. H. C. Dahe
THE ART OF GEM CUTTING, 3D ED.
Dedicated
in gratitude and affection to
Lillian Drews Garvin
t
HOW TO KNOW THE MINERALS AND ROCKS
Copyright. 1955. by Richard M. Pearl. All rights in
this book are reserved. It may not be used tor dra·
matico motion·, or talking.picture purposes without
written authorization from the holder of these rights.
Nor may the book or parts thereof be reproduced in an}
manner whalsoever without permission in writing. ex·
cept in the case of brief quotations embodied in critical
articles and reviews. For information, address the
McGraw-Hili Book Company, Inc., Trade Department,
3110 Wesr 42d Street, New York 116, New York. Library
of Congress Catalog Card Number: 511-5785'
PUBLISHED BY THE MC CRAW-HILL BOOK COMPANY, INCPRINTED IN THE UNITED STATES OF AMERICA
ElCHTH PRINTING
Preface
How to Know the Minerals and Rocks is a practical field
guide to more than 125 of the most important minerals
and rocks. including gems. ores. native metals. meteorites,
and other interesting members of the mineral kingdom.
It is simplified. authoritative, and up to date-written for
the layman and for the beginning and amateur collector.
With it you can identify for yourself the better-quality
typical specimens you are most likely to find, as well as
others of outstanding interest to collectors, prospectors,
and scientists.
Even many fairly advanced collectors of minerals are
unable to recognize by name the commonest rocks. because of their diverse appearance and the lack of definite
tests for them. The Four Keys to Recognizing Rocks
make it easy to identify the chief types by a simple but
systematic procedure.
Similarly. the Seven Keys to Recognizing Minerals enable the new collector to become acquainted quickly with
the important minerals which make up the majority of
those he will come across in this fascinating and fastgrowing hobby. Moreover, no special skill is needed to
make the tests, and no equipment other than a pocketknife. a common magnet, a piece of broken porcelain, a
piece of glass, a copper coin, a piece of quartz, and some
vinegar.
Although the author believes wholeheartedly in the
value of the standard blowpipe methods for testing minerals and has taught them to many students, they are not
employed in this book because of the proved reluctance
of most collectors to attempt their use without personal
instruction. A few other kinds of tests are mentioned, but
the minerals can be identified without them.
Besides the Keys for mineral and rock identification,
this book has more unique features. Each description of a
mineral and rock is accompanied by a drawing which
brings out clearly the typical appearance and the characteristics by which the mineral and rock can be recognized.
These well-labeled drawings, based upon sketches that
were made especially for this book by the author's wife
and were prepared in close collaboration with his writing
of the text, will prove more valuable to the collector than
pages of explanation.
The description of certain minerals includes handy tips
on collecting, handling, cleaning, preserving, or displaying them-information not readily available elsewhere.
The descriptions also cover the chemical composition.
occurrence, uses, historical lore, and a range of other entertaining and informative background material on each
mineral and rock.
Simplified information is given for learning how to
read the chemical formulas of minerals. A list is presented
of all the national magazines in the United States devoted
to mineral and rock collecting and related hobbies. A selected reading list of books on these subjects is given, with
brief descriptions to aid you in purcbasing them.
Other distinctive features of this book include careful
attention to scientific words so that all su(,h words are explained when first used; emphasis of technical words by
italics; thoughtful selection of photographs to tie in with
the text and illustrate the discussion; and a complete
Index.
Richard M. Pearl
COLORADO SPRlNGS, COLO.
Contents
Preface
Acknowledgments
CHAPTER I
Th is Fascinating Mineral Hobby
CHAPTE1t.2 How Rocks and Minerals Are Formed
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks
Soils and Scenery
CHAPTER lJ
Building a Mineral Collection
CHAPTER 4 Seven Keys to Recognizing Minerals
MINERAL KEY NO. I
Luster
MINERAL KEY NO. 2 Hardness
MINERAL KEY NO. ~ Color
MINERAL KEY NO. 4 Streak
MINERAL KEY NO. 5
Cleavage
MINERAL KEY NO. 6 Fracture
MINERAL KEY NO. 7 Specific Gravity
Other Mineral Properties
How to Read Chemical Formulas
How to Use the Outlines
Identifying the Minerals. Outline Of Keys
CHAPTER 5 Four Keys to Recognizing Rocks
ROCK KEY NO. I
Texture and Structure
ROCK KEY NO.2 Color
ROCK KEY NO. ~ Acid Test
ROCK KEY NO. 4 Mineral Content
Identifying the Rocks. Outline of Keys
Magazines fOT the CollectOT
Books fOT the Collector
Index
9
17
45
46
47
47
51
5~
54
161
161
162
162
163
164
187
188
lSg
Acknowledgments
In addition to preparing preliminary sketches for aIm t all the drawings, my wife, Mignon W. Pearl, has
given careful attention to the rest of the manuscript, and
her advice has been extremely helpful. Eva L. Keller, of
Colorado Springs. designed one of the difficult illustrations. Dr . Don B. Gould, of Colorado College, helpfully
supplied a needed item ot information. Nordis Felland,
Librarian of the American Geographical Society, New
York., check.ed some geographic names.
Stephen ) . VooThies prepared the line drawings that
appear throughout this book..
I wish to express my gratitude to the following for color
illustrations : Henry L. Gresham of Ward's Natural Science Establishment, Rochester, N. Y. , who made available
the photographs of minerals from the Harvard Universi ty
Collection ; Harry B. Groom, Jr., Assistant Professor of
Geology, Louisiana Polytechnic Institute. whose photographs of minerals in the Harvard Collection appear on
the jacket; and Lelca Photography Magazzne , in the pages
of which Professor Groom's photographs were first reprod uced.
R.M.P.
CHAPTER 1
This Fascinating M ineral Hobby
Collectors of minerals and rocks are rapidly becoming
more numerous all over the world, especially in the
United States and Canada. This has been true for nearly
twenty years, yet mineral collecting is still young and
vigorous enough to offer rewarding opportunities for
those who join the fastest·growing collecting hobby in
America.
Enthusiastic "rock hounds" are to be met today in practically every community. Tens of thousands of adults and
youngsters have been attracted to this exciting activity
within recent years. You may become a member of a mineral or gem society in almost every state and province, attend regular meetings. and go on conducted field trips to
obtain specimens fro m many interesting localities. Over
ninety such clubs exist in California alone, many of them
providing junior memberships for boys and girls to encourage them in this wholesome and profitable hobby.
Most local and state societies are banded together into
one of the six regional federations-Eastern, Midwest,
Rocky Mountain, Southwest, California, and Northwestwhich in turn are affiliated with the American Federation
of Mineralogical Societies. These federations sponsor an·
nual conventions, which attract a large attendance to see
the extensive exhibits of fellow collectors and dealers who
display the newest discoveries and latest equipment. Such
a convention is a thrilling spectacle. And there are thousands of equally ardent collectors who do not belong to
an organized group but enjoy hunting rocks and minerals
just the same.
A hobby that is expanding this fast, appealing to people of all ages and occupations, must possess some strong
points. Indeed, mineral collecting does have exceptional
advantages to recommend It.
First of all, it is carried on primarily out of doors, where
you become acquainted with the wonders and beauties
9
of Nature at her best and learn to understand the expressive face of the earth in its manifold aspects. Scenery is
ever-changing, in response to the weathering and erosion
of minerals which constitute the rocks of the earth's crust.
We find out from a study of minerals why cliffs wear down
and how soils originate, why the walls of the Grand Can·
yon show such vivid hues, why the sands of the Florid a
beach are so varied in size and sha pe and color. As the
English art critic and author John Ruskin wrote,
"There are no natural objects out of which more can be
learned than OUt of stones. They seem to have been created especially to reward a patient observer. For a stone,
when it is examined, will be found a mountain in mini·
ature. The surface of a stone is more interesting than the
surface of an ordinary hill, more fantastic in form, and
incomparably rIcher in color."
KEMBER
FIg.
I
Emblem of American Federation of
Mineralogical Societies
Mineral collecting, furthermore, has its indoor opportunities, even for the shut-in, who can acquire desirable
specimens by trading with other collectors and buying
from dealers. Splendid selections of minerals are prominently shown in museums in most of the larger cities and
in numerous smaller ones, as well as in many colleges and
universities. No more pleasant way to spend an evening
can be found than in examining the collections of others
in your own community or while traveling.
The art of amateur gem cutting, beginning as an off10
Fig.
2
Faceted and cabochon gem cuts
shoot of mineral collecting, has become a major hobby
in itself as thousands of home craftsmen are attaining results superior to those of commercial lapidaries, because
they are eager enough to experiment and persevering
enough to bring their work to a high degree of perfection.
A few of them, endowed with the gift of artistic expression, turn out carvings of surpassing beauty. Some of
them are able to cut faceted stones, having flat surfaces or
"faces" at different angles, while others prefer to make
cabochons, which are simpler because they need only a
rounded top. Enterprising boys, and girls too, using inexpensive equipment, have developed a skill equal to
that of their elders. If you would like to transfonn stones
into flashing gems, here is the hobby for you.
BRILLIANT CUT
Fig. 3
MARQUISE
DC
EMERALD CUT
BAGUETTE
Popular faceted cuts of gems
Crystals, which Abbe Haiiy justly called "the flowers
of the minerals," are the chief delight of a large proportion of mineral collectors. Smooth and shining faces,
bright hues, and intriguing forms combine to make crystals outstandingly interesting to those who admire beautiful things. Though scarcely any two crystals seem alike,
we are able to classify them all into six types known as
crystal systems. These are named below, with a model of
11
ISOMETRIC
TETRAGONAL
HEXAGONAL
Galena
Chalcopyrite
Apatite
ORTHORHOMBIC
MONOCLINIC
TRICLlNIC
Orthoclase
Rhodonite
Feldspar
Fig.4 The six crystal systems
A common crystal form or combination of forms is
shown for each system. Countless modifications are
possible.
Sulfur
an important mineral belonging to each system. They
differ from one another in the length and arrangement of
the axes which run through them; the axes are only
imaginary, like the equator and poles of the earth, but
are extremely useful in describing crystals.
The most perfect crystals are the smallest ones, because
they have been protected by their very smallness. A collection of such miniature crystals or micromounts, which
are tiny clusters delicately mounted in a box and viewed
through a magnifying glass or microscope, reveals a fairyland of breath-taking sparkle and color.
Another phase of mineral collecting that can be carried
on indoors is the growing of crystals from saturated chem12
ical solutions. You may cause the crystals to change shape
or color in surprising ways by adding a drop of acid or
otherwise interfering with the solution as it gives up its
dissolved matter.
Few collectors try to accumulate a sample of every mineral or rock; most of them leave to the large museums the
task of gathering a comprehensive general collection.
Within a short time the beginning collector usually finds
out what kind of specimen he likes best and then concentrates on that kind. The most distinctive collections are
made by limi ting the scope of your efforts and focusing
on those that have the greatest appeal to you_
Fig. 5
Micromount, to be observed under magnificatIon
For instance, a collector may specialize in a single mineral-perhaps calcite, which comes in more than 300 different crystal forms; or perhaps quartz, which is abundant everywhere and is found in astonishing variety. Another collector may lean heavily toward ore minerals, of
interest particularly to miners and prospectors. Ores of
common metals such as iron and copper may be emphasized, or of precious metals such as gold and silver. Minerals from one state or region (perhaps the Rocky Mountains, Death Valley, or New England) or from your home
county; minerals of your favorite color; minerals that are
finely crystallized; or gem minerals-these likewise are all
worthwhile subjects for specialized collections.
An enviable collection might also feature unusual occurrences of minerals, such as geodes, which are nodules
lined with crystals; or altered minerals called pseudomorphs (petrified wood is a good example, the original
wood having turned to stone); or sand from rivers and
13
Fig. 6
Geode from Brazil, showing inside lined with
amethyst crystals
seashores, or stalactites from caverns, or meteorites which
reach the earth from the mysterious vastness of cosmic
space. Any of these and many others can be obtained by
finding, trading, or buying them. Rocks, either typical
ones or curious freaks, and fossils buried in the rocks are
appropriately included in a mineral collection. At one
time, in fact, minerals and rocks were both called fossils,
which in Latin meant "to dig," because they are taken
from the earth.
Fig. 7 Stalactite from
Mammoth Cave, Kentucky
Mineral collecting can be more than outdoor or indoor
fun. It can lead to an acquaintance with one of the most
vital sources of human wealth. Mining ranks with farm·
ing, fishing, and lumbering as a primary producer of basic
raw materials. Man has become dependent upon the min·
eral industries for the existence of both his peaceful and
his military civilizations. Yielding metals, nonmetallic
14
substances, and fuels, our mineral resources largely create the conditions of present-day life on this planet. In
addition, minerals have always played an important part
in the development of chemistry, physics, and other
sciences.
By exposure to the weather, rocks and their minerals
decay and fall apart to become soil. Plants are thus able
to grow, in turn providing food for animals. Water, which
is the most essential of all foods, is also an integral part of
the mineral kingdom, being either a rock or a mineral,
according to even the most precise definitions.
It is hard to decide which are the most important mineral products besides soil and water. Salt must be included, because it is a mineral indispensable to life-the
location of salt licks has marked the route of caravans
throughout the centuries. The lure of gems has encouraged
trade and transportation since the dawn of history. Flint,
a member of the quartz group, was highly prized forweapons, which became the first manufactured articles. Clay,
used for pottery, started the earliest large-scale mineral
industry. Building stones, as were used in the pyramids,
contributed a good deal to the expanding service of earth
materials.
Those mentioned above belong among the nonmetallic
or so-called industrial m inerals and rocks. The list is
seemingly endless, hundreds of them being used in thousands of ways. Consider some more of them-pumice from
the Lipari Islands to polish your teeth or from California
to insulate your home; potash from New Mexico to fertilize the soil and make farming a scientific employment;
emery from Greece to grind away metal in an airplane;
asbestos from Quebec to be woven into fireproof clothing;
feldspar from North Carolina to glaze chinaware; mica
from South Dakota to be flaked into Christmas-tree
"snow"; sulfur from Texas to make possible the heavychemical industry. Specimens of them all should be represented in your mineral collection.
Coal and petroleum are mineral fuels. Though not so
attractive to most collectors, their importance should not
be overlooked. Coal is the very foundation of modern industrial economy; countries that lack ample reserves of
15
suitable coal cannot hope to do a large share of the
world's manufacturing. Armies, navies, and air forces
ride, float, and fiy to victory on petroleum.
The metallic minerals are the ores from which metals
are extracted. These metals may be gold, silver, or platinum-the precious metals; or copper, lead, or zinc, which
are known as base metals; or iron and the so-called ferroalloy metals, which are mixed or alloyed with iron to
make steel. Discovery of the art of smelting ores revolutionized man's life, enabling him to obtain useful metals
from otherwise worthless rock. He could then also melt
two or more metals together to produce brass, bronze, and
other alloys.
So significant are the mineral products to civilized
beings that human history itself is divided into the Old
Stone Age, the New Stone Age, the Bronze Age, and the
Iron Age. Perhaps we have now moved into the Uranium
Age-only the future can tell. As a mineral collector you
will be playing your part in the thrilling drama of man.
The amateur collector will probably not come into the
fortune of a Texas oilman or of Dr. Williamson, the African diamond magnate, whose faith in his knowledge of
the rocks during years of fruitless search was finally repaid
by his discovery of the world's largest diamond mine.
Nevertheless, the amateur will surely find many fine minerals, perhaps some of satisfying value, and possibly even
a new one unknown to science. He will at the same time
learn more about his natural environment. Such knowledge is one of the marks of a broadly educated person.
And in doing so he will vastly enjoy himself in this vital,
many-sided hobby.
More frequently than in the past, hobbyists are turning mineral collecting into a business, selling at a profit
to museums, other dealers, and private collectors. Occasionally someone does this as a full-time vocation. The
personal experiences of capable professional collectors
such as Edwin W. Over, Jr., of Woodland Park, Colo.,
and Dr. Harvey H. Nininger, the Arizona meteorite expert, in out-of-the-way places ranging from Baja California to Prince of Wales Island off the coast of Alaska,
would make entertaining adventure stories.
16
CHAPTER 2
How Rocks and Minerals Are Formed
The difference between a rock and a mineral should be
clearly understood. Rocks are the essential building rna·
terials of which the earth is constructed, whereas minerals
are the individual substances that go to make up the
rocks. Most rocks. therefore, are aggregates of two or more
minerals. Thus, granite (a rock) is composed of at least
two minerals (quartz and feldspar), though others are
almost certain to be present.
If a single mineral exists on a large enough scale, it may
also be considered as a rock, because it may then be re·
garded as an integral part of the structure ot the earth.
Thus, a pure sandstone or quartzite rock contains only
one mineral, quartz, distributed over a wide area. Other
single minerals which are described in this book and are
regarded also as rocks by this definition include anhy·
drite, dolomite, gypsum, magnesite, serpentine, and sultuT
-all ot which occur in huge beds or masses. Some rocks ot
this type have a different name from that of the mineral
composing them. Thus, the mineral halite makes rock
salt; calcite is the constituent of the rock called limestone;
and either calcite or dolomite can make up the rock called
marble. Kaolinite composes many of the rocks we know as
clay. Bauxite has been proved to be really a rocky mix·
ture of several minerals, but many geologists still preter
to call it a mineral.
In addition to these two classifications, rocks include
natural glass, though it may be devoid of any actual min·
eral components. Obsidian, an abundant rock in Mexico
and Iceland, is natural volcanic glass. Organic products
of the earth, which cannot be called minerals because they
are formed from plants and animals, are properly known
as rocks. Coal, derived from partly decomposed vegeta·
tion, is a rock of this kind.
Seldom will you find a single species of mineral occur·
ring entirely by itself. Like people, minerals have a tend·
17
ency to be found in the company of others ot the same
kind, having formed under the same conditions. This is a
fact which proves most helpful to the collector, who soon
discovers that often the best way to recognize a mineral is
by its associations.
Thus, feldspar and quartz occur together in the rock
called pegmati te because they originate in the same manner, that is, by the cooling of molten rock of a certain
chemical composition and within the limited range of
temperature required to form pegmatite. Again, no one
can fail to know at a glance that he has a specimen of the
zinc ore from Franklin, N. J., when he sees the distinctive
combination of red zincite, yellowish-green willemite,
black franklinite, and white calcite. These minerals are
not found together anywhere else in the world, and each
mineral immediately suggests the presence of the others.
As another instance, in 1870 a man named DeKlerk was
led to the first diamond ever recovered from its original
rock when he saw some pebbles of garnet in a dry stream
bed in South Africa and realized that the two gems often
occur side by side.
Moreover, each group ot minerals is related naturally
to definite types of rock. This enables us to identify the
rock more readily than otherwise. Rocks are not so easy
to name as minerals because they grade imperceptibly
into one another, but this principle of mineral association
is very helpful.
The many rocks which constitute the earth's crust are
the result of geologic processes acting during long ages,
building up some rocks and breaking down others. The
normal rock cycle leads from molten rock to igneous rock,
then to sediment and sedimentary rock, followed or preceded by a metamorphic stage. Countless bypaths to this
cycle give rock an infinite variety and prevent them from
becoming monotonous to anyone who has gained a speaking acquaintance with them and even a slight knowledge
of geology.
Igneous Rocks
All minerals and rocks have their primary ongm in a
body of molten rock called magma, which is believed to
exist in local pockets deep within the crust of the earth.
This magma eventually becomes the igneous rocks and
minerals. The name igneous, related to the word ignite,
suggests fire and heat.
Seismologists, who are the scientists who study earthquake waves, tell us that the earth beneath its relatively
thin surface layers is not liquid, as it was formerly thought
to be, nor is there outside the core a zone of molten
rock. Probably the hot rock is prevented from melting by
the enormous pressure upon it, which maintains it in a
semiplastic condition. When the pressure is relieved anywhere by cracks in the solid rock above, or heat due to
radioactivity reaches the melting temperature, the rock
slowly begins to rise in a molten state.
As this magma comes to rest in a cooler place, but still
within the earth's crust, it starts to solidify; and thus the
igneous rocks are born. They are known as intrusive rocks
because they have intruded or forced their way into other
rocks which were there already. This process has been
going on ever since the beginning ot geologic time, and so
igneous rocks are presumably being formed in the same
way today as they have been throughout the long history
of our planet.
The intrusive igneous rocks common and important
enough to be described in this book are porphyry, granite,
pegmatite, syenite, monzonite, gabbro, and peridotite.
Constituting the core of mighty mOllntain chains, these
rocks are revealed for observation only after millions of
years of prolonged weathering and erosion by the wind
and rain and other agents of the atmosphere.
When the molten rock actually breaks through to the
surface and wells out as a lava flow, or is blown out as
volcanic fragments. the resulting igneous rock is called
extrusive. We usually have in mind a volcano such as
Vesuvius or Mauna Loa when we refer to this sort of
19
~
igneous rock, but lava can issue qUletly trom open fissures
in the earth without building up a cone or crater, as it
still does in Iceland.
More than 500 volcanoes have erupted within recorded
history. It is hardly safe to say that a volcano is extinct,
for it may only be dormant, waiting for an occasion to
awaken once again. Lassen Peak, sleeping in the California Sierras, surprised the nation in 1914 by becoming the
only active volcano in the United States. When Vesuvius
sprang to life in A.D. 79, suffocating Pompeii and Herculaneum in its grip, it had been so long quiescent that its
old activity had been forgotten by the Roman people.
Although volcanoes are widespread throughout the
world, the most striking feature of their distribution is
the so-called ring of fire surrounding the Pacific Ocean
basin, from the tip of South America north to Alaska and
back down to New Zealand. Another belt of volcanoes
roughly follows the equator from the West Indies to the
Mediterranean and on to the East Indies.
The extrusive igneous rocks included in this book are
obsidian, pumice, felsite, and basalt. Formed upon the
surface or at a shallow depth beneath a light covering,
these rocks need not wait long before weathering and
erosion set in upon them. The term vulcanism, however,
includes the behavior of all molten rock, whether it takes
place on the surface or far below and whether or not it
builds up a typical volcanic mountain with an opening in
the center. The originator of this extensive activity
was thought to be the Roman god of fire, Vulcan, who
operated hIS workshop underneath glowing Etna, one of
the natural lighthouses of the Mediterranean.
/ Intrusive and extrusive igneous rocks are unlike chiefly
because they have cooled at different rates. Intrusive
rocks, losing their heat slowly while beneath the ground,
acquire a coarse texture as the individual minerals have
time to grow to a considerable size. Observe, for instance,
the conspicuous pink feldspar. white quartz, and black
hornblende in the granite from Pikes Peak. The slowestcooling igneous rock is pegmatite. and its constituent minerals may be enormous-single crystals of spodumene 20
feet long and muscovite mica 10 feet across.
20
On the other hand, extrusive rocks cool rapidly; many
grains get started, but each is small. Compare even the
normal texture of granite, as described above, with the
dense basaltic lava of the Columbia Plateau in Oregon,
Washington, and Idaho.
In extreme cases of sudden chilling, no minerals are
visible at all, the only product being a natural glass. Obsidian Cliff in Yellowstone National Park. seen by a million tourists annually, is a world-famous example of volcanic glass which originated in this fashion. A porous texture, especially of pumice, results from the escape of gas as
it bubbles into the air.
• Igneous rocks, such as granite and felsite, that are rich
in silicon tend to be light-colored and relatively light in
weight. As the amount of silicon is reduced and the proportion of iron and magnesium increases. the igneous
rocks become darker and heavier, as gabbro and basalt
are.
The cooling of magma to form an igneous rock is accompanied by shrinkage and the development of parallel open cracks called joints.
Shrinkage also causes cavities or pockets. and these may
later be filled or lined with crystals projecting toward the
center. Some of the man-sized pockets of this sort yield
large gemmy crystals of quartz, feldspar. and other minerals. •
Another phase of igneous activity that concerns the
mineral collector has to do with ore deposits. Metal-bearing solutions of many kinds accompany the rise of magmas. As the molten rock cools and becomes solid, large
quantities of liquid and gas, charged with mineral matter, are given oft. Leaving the igneous rocks behind them,
they make their way slowly toward the surface, forming
mineral deposits wherever conditions are favorable. Thus,
lower temperature, reduced pressure, the presence of
limestone and other easily changed rocks are conducive
to the deposition of ore minerals.
During the long-distance migration of the solutions
that have been expelled from the magma. ore deposits of
gold, silver, lead, zinc, and other metals are produced_
These are referred to as veins because they run through
.u
Fig. 8
Cross section through the earth, showing bodies
in which igneous rock occurs
the enclosing rock like veins within the skin. They are
classified according to the temperature and pressure at
which they came into existence, which in turn depend
upon the distance they have traveled from the magma.
Eventually, if not used up by one of the processes just
described, the mineral matter that is left may appear at
the surface of the earth in a volcano, gas vent, geyser. or
hot spring. Around the volcanoes of the Mediterranean
shore. for instance, are coatings of such minerals as native
sulfur. realgar. and hematite, which are described in this
book. Amidst the spectacular fumaroles or gas vents of
the Valley of Ten Thousand Smokes in Alaska are magnetite. pyrite, galena, and other minerals in large
amounts. The geysers of New Zealand carry gold, silver.
and mercury. Hot water at Steamboat Springs, Nev., is
depositing cinnabar today as in the past. There are
numerous similar examples of each of these mineral occurrences, representing the final stages of igneous activity.
Sedimentary Rocks
Even the most deeply buried igneous rock may someday
be exposed by erosIOn. The age of the earth, determined
by the measurement of radioactivity in igneous rocks to be
more than three and one-half billion years old, allows am- .
pIe time for very extensive erosion to have occurred al·
Ia
most everywhere. The forces of weathering will then
begin to attack the rock, causing it to crumble and decompose. Some of the fine particles may be dissolved by
rain water as it seeps through the soil and into the pores
of the bedrock underneath. The rest may be washed away
bodily by streams, or wafted by the wind, or carried in the
frozen grip of giant glaciers.
When either the dissolved rock matter or the transported sediment is deposited somewhere else and afterward hardens into firm rock, we have a sedimentary rock,
the second of the two major kinds of rock. Two types of
sedimentary rock are possible, according to whether the
original material had been dissolved in water or had been
moved in the form of fragmen ts.
In the first case-represented by such rocks as rock salt
in Kansas and Michigan and beds of borax in Death
Valley, Calif.-the minerals are deposited when the dissolving power of the solution is reduced. This may happen because the water gets cooler or some of it evaporates
or because of the action of certain plants and animals
which extract chemicals from the water. In the same
fashion sugar settles at the bottom of the cup when coffee
cools, and salt incrusts the sides of a pan when salty water
or brine is evaporated.
The second type of sedimentary rock is built up by the
accumulation of separate grains of mud, sand, or gravel.
Thus, mud becomes shale, sand becomes sandstone, and
gravel becomes conglomerate. These sediments vary considerably in their mineral composition, and they grade
into one another in the size of their particles.
Although the importance of wind and glaciers as transporting agents cannot be denied, most sedimentary material is nevertheless carried by streams. Rivers are therefore not only the great sculptors of the landscape and the
chief creators of scenery, but they likewise play the major
role in transporting the products of the earth that are to
become the sedimentary minerals and rocks.
Probably the most intriguing sedimentary deposits are
the ones known as placers, in which are concentrated
gold, gems, and other heavy, durable minerals. The
bearded Western prospector, equipped with gold pan and
as
accompanied by his faithful burro, is the symbol of placel
mining. Resisting chemical decay and physical damage
alike, heavy minerals that will end in placers are washed
from the higher elevations and taken downstream, until
the force of the water is no longer sufficient to move them
any farther. A slight obstruction in the channel or change
in the current may be enough to cause them to drop to the
bottom.
Billions of dollars' worth of native gold has been recovered from the bonanza placers of California, the Klondike, and elsewhere. Besides gold and a number of valuable gem stones-such as diamond, corundum (ruby and
sapphire) , spinel, and zircon-the most likely constituents
of placers include magnetite, chromite, ilmenite, and cassiterite. Quartz, of course, is ever-present.
A special kind of placer is laid down along ocean
beaches by waves and shore currents, which effectively
separate the heavy minerals from the light ones. At Nome,
Alaska, two submerged beaches and four now elevated
above sea level have yielded a good deal of gold in very
tiny grains. Vast accumulations of ilmenite, rutile, and
zircon line the beaches of India, Brazil, Australia, and
Florida. So-called black sands, containing magnetite,
ilmenite, and chromite, are extensively developed on the
coasts of California, Oregon, and Japan. A most extraordinary representative of beach placers is the rich diamond bed near Alexander Bay in Namaqualand, South
Africa, where diamonds brought down by the Orange
River were distributed along the beach, in dose association with oyster shells.
Wind often blows the smaller rock fragments into
heaps called dunes. A single sample of dune sand may
contain several dozen different minerals, but they are
not of specimen interest except to collectors of sand who
must study them under magnification.
Owing to their bulk, glaciers are effective agents in
transporting and depositing sediments. Unlike streams
and wind, they are not selective in their action, so that a
glacier embraces in its icy grasp boulders the size of a
house, surrounded by particles that have been ground so
felentlessly as to deserve the name "rock flour." With
24
equal disregard for size, a glacier dumps the large and
small material at the same time, with no attempt at sorting it. Such accumulations. common in every area glaciated during the recent Ice Age, are called moraines. Incorporated in moraines are minerals and rocks of foreign
extraction which have been pushed, dragged, or carried
bodily from their place of origin. in some cases hundreds
of miles away. Chunks of native copper. brought down
from the Upper Peninsula of Michigan, are strewn from
southern Iowa to Ohio. Masses of chalcocite are frozen in
a moraine at Kennecott, Alaska.
Fig. 9
Stratification (horizontal) and joints (vertical)
in sedimentary rock , limestone in Indiana
Loose sediment, whatever its origin, eventually becomes
solid-"as hard as a rock" -because mineralized underground water cements together the individual grains and
the weight of later sediments squeezes down upon them
more and more tightly. At a fairly shallow depth, except
in arid climates, the ground is saturated with water which
fills all pore spaces of the soil and bedrock. This water
drains into streams or soaks out at the surface as seeps
and springs. In caverns stalactites hang from the roofs,
while stalagmites build up from the floors-both the result of evaporation of underground water as it percolates
into the earth.
The distinctive property of most sedimentary rocks is
their srr.atincaljo~, which refers to the layers or beds as
each one is deposited on top of the earlier ones. Just as
.25
TRILOBITE TOOTH OF ANClENT<flHARK
Fig.
IO
PELECYPOD SHELL
Common fossils. in sedimentary rock
the bottom book in a pile must have been the first one
put down, so the lowest bed was the first one deposited,
and each successive bed was formed at a later time.
Only in sedimentary rocks are fossils common. Obviously the heat of an igneous rock would consume any evidence of animal or plan t Hfe that might have existed.
Another characteristic of sedimentary rocks is the presence of foreign lumps or nodules called concretions. In
the white cliffs of Dover are numerous odd-shaped pieces
of tlint, perhaps secreted by ancient sponges when the
chalk that now makes up the cliffs was deposited in a
shallow sea.
Fig.
Concretion from
shale bed, Virginia
I I
Joints are as abundant in sedimentary rocks as they are
in igneous rocks, but the shrinkage which causes them is
the result of drying instead of cooling. Most sediments
are laid down in water-streams, lakes, the ocean-and
26
may contain 50 per cent or more of moisture, some of
which is driven off during buriaL
The sedimentary rocks described' in this book are conglomerate, coal;"shale, limestone; and sandstone.
Metamorphic Rocks
The third main kind of rock, called metamorphic, is the
result of drastic changes in either an igneous or a sedimentary rock. The new rock has been changed so much
nom Its original state that practically all signs of ancient
life are gone, and so fossils are almost never present. Even
most of the evidence as to the nature of the original rock
has been lost, aEd often it is impossible to tell whether
the preexisting rock was igneous or sedimentary.
eat from an invading magma that forces its way
towar the surface of the earth is one of the factors that
produces a metamorphic rock by creating new textures
or entirely new minerals from the old rock. Another
factor is ressure resulting from deep burial or slow
movement in the earth's crust, pressure of the sort that
ultimately bends rocks into mountain ranges. The chemical action of liquids and gases is also effective.
Limestone, for example, turns from a sedimentary rock
into marble as the grains ~alci~ recrys~ize under. the
inBuence of the agents 0 metamorphism. Because of the
recrystallization, marble usually has a more glistening appearance than limestone. New minerals may be fonned in
the process, giving marble the swirled patterns that are so
attractive a feature of colored marble. The collector may
expect to find garnet, idocrase, and epidote, among many
other minerals characteristic of this type of occurrence.
Scheelite, an ore of tungsten, is perhaps the most valuable
metallic mineral originating in this manner.
Joints are prominent in metamorphic rocks, for the
granual but irresistible application of natural forces
strains them past the breaking point.
The metamorphic rocks described in this book are
gneiss, s~ slate, marble, and uartzite.
Soils and Scenery
The weathering of any of these three major types of rock
leads to the formation of soil. In addition to broken fragments of decomposing rock, soil consists of decayed
plant material. With continued weathering, the soil zone
becomes deeper and the bedrock further concealed. New
minerals are created in this process, but with few exceptions they are scarcely suitable as specimens. Some of the
chemical elements in the soil combine into various soluble compounds which, when they are washed out of the
ground, leave the soil depleted of elements essential to
plant growth.
As important to the natural history of rocks and minerals as the making of soil and the production of ore
deposits is the creation of the scenery that beautifies and
diversifies our terrestrial environment. The inhabitants
of the earth during much of geologic time-if there had
been any thinking beings prior to our own brief agewould have lived on a monotonous globe. Often devoid
of mountains and hills, glaciers and volcanoes, the earth
throughout a large part of its existence has been a desolate place. When the seas inundated the low-lying continents, they spread widely over the dreary landscape.
When they retreated, they left behind an even more barren scene. At irregular intervals, of course, majestic mountain ranges pushed their way into the sky, thick masses of
glacial ice inched precarious paths down the frosty slopes,
and volcanoes spewed forth flaming debris upon the
horizon.
But these momentous events only made the dismalness
of the intervenir:g periods seem the more uninspiring by
contrast. We are more fortunate than perhaps we deserve
to live in a world only lately having suffered the paroxysms of mountain building, only recently emerging
from an Ice Age which has crowned its higher elevations
with shining diadems of white.
What scenery comes into view in any given place at
any given time depends upon the underlying rock and
.28
I
HORIZONTAL BEDS
Fig.
I2
TILTED BEDS
Difference in topography caused by structure
the kind of geologic agent acting upon it. Hard limestone,
for instance. may stand erect as a sharp cliff, while a
crumbly shale next to it will disintegrate into a rolling
valley. Whereas flat layers of durable sandstone will protect from erosion the softer beds between them, thereby
forming a mesa or butte, the same association of rocks
will appear as alternating hills and valleys if the layers
have been tilted on edge.
Blowing sand fashions scenery in a desert region unlike
that of underground water in an area noted for its caves.
Ocean breakers are responsible for scenic effects very dif·
ferent from those caused by the pull of gravity as it sets
into motion landslides and avalanches.
YOUTH
MATURITY
Fig. I!J
OLD AGE
Stream cycle of erosion
Even the same agent of erosion or deposition avoids
uniformity by taking its scenery through a cycle. Rivers,
for instance, go from a youthful stage to one of maturity,
then into old age. As the cycle progresses. the valley assumes new characteristics which enable the experienced
observer to recognize the relative age of that particular
stream. The sharp V- haped valley of a youthful stream
gives way to one having a flatter profile as the river cuts
its way downward and sideward. The development of
29
meanders accentuates the swing of the stream until, in old
age, it wanders across a wide valley between low hills.
To the person with a vivid and inquiring mind, the
rocks and the minerals in them are never static things.
They come into the earth's crust in numerous ways, they
are transformed into many new substances, and they
undergo experiences that challenge the imagination. And
they never cease to exist in one form or another. To such
an alert person who also knows the dramatic story of the
rocks and minerals, they are not only seen in three dimensions and in color but they acquire the fourth dimension of time.
CHAPTER 3
Building a Mineral Collection
A mineral and rock collection is as near as your back
yard and as distant as the far corners of the world. Specimens of some members of the mineral kingdom may be
secured with little trouble and less cost, while others
require the initiative of a globe-trotter and the purse of a
merchant prince. Minerals are to be found in garden soil,
in road cuts and building excavations, under cliffs and in
stream beds, on beaches and sand dunes, in quarries and
mines. Heaps of waste rock called dumps are a prolific
source of specimens. Everywhere around us, in fact, minerals are present.
Cavities in rock are especially favorable spots for finding choice crystals, for in such open spaces they have a
chance to grow freely to substantial sizes. Ordinary-looking boulders known as geodes, when broken open, often
reveal rows of glistening crystals lining the inner walls,
like the inside of a treasure cave in a storybook. Mammoth crystal-filled geodes, outwardly resembling the eggs
of some prehistoric beast, are picked up by the thousands
around Keokuk, Iowa, and elsewhere in the central Mississippi Valley.
A small number of minerals can be sifted as pebbles
from the soil in one's own yard. Sand and clay pits and
other diggings in soft rock will yield additional interesting minerals, among them fine specimens of gypsum, pyrite, and marcasite. Near exposed cliffs and rock outcrops,
road and railway cuts and tunnels in solid rock, and excavations made for the construction of buildings are
promising places to search for minerals. Quarries and coal
mines are common, and their waste piles may be surprisingly prod c~j e, A,n inttiID!in feat:ure of ;oal deposits
is the abuQda~ce o! fos~i1s-~e !em~i~~ or 1 pressions of
plants and a~l1mals that once- lIved ~n the arth. Monument work, wliith ' re often sittiated in 'th~ vicinity of
1I vjsit., and so are 'Smelters.
quarries, a e wo.,w,
·,I,~. r
.'
_
•
.'
~
Sl
-
,
"'.' J ~~ _ ...... 1
The best sources of minerals are, of course, mines and
their dumps. In the United States, mines are not limited,
as you may have believed, to the open spaces of the West.
On the contrary, Pennsylvania, West Virginia, Illinois,
Kentucky, and Ohio stand among the eight leading mineral-producing states. The second-largest zinc mine in the
country is near Ogdensburg, N.J., within 40 miles of the
metropolis of New York City, and the fourth-largest is
in St. Lawrence County, N.Y.
The loose sand and sagebrush areas of deserts yieldbesides agate and other minerals usually expected there
- flowerlike groups of crystals, including the attractive
"desert roses" and " barite roses." The temporary lakes in
arid regions give up halite, borax, and many other saline
minerals, which form when the water evaporates. Beach
sands may contain a multitude of heavy minerals brought
together by waves and currents from widespread locali·
ties. Owing their origin to the uplift of a mountain range,
the explosion of a volcano, the flow of lava, the bubbling
of a hot spring, even the crash of a shooting star, the
products of the mineral kingdom are on every hand.
Mineral localities consequently are world-wide. Many
specimens are available on pu blic land, especially in the
national forests ; the national parks and monuments, except Death Valley, are closed to collecting. Others can
often be obtained on private property by asking permission and taking care to close gates and respect the rights of
the owners or residents.
You might consider seriously the size of the specimens
you want to collect. Some collectors, having unlimited
space, feature large and showy specimens, whereas others
are obliged to restrict themselves to micromounts, the exquisite miniature crystals which occupy little more room
than a stamp or coin collection. Either extreme is perfectly acceptable, though the average collector acquires
whatever pleases his fancy, regardless of size, and seems al·
ways to need more room than is available. The number of
specimens of the same kind to be taken is also a matter of
decision. It is true that duplicate ones can usually be
traded off, and even sold, but it is the excessively greedy
S~
collector who gives the hobby a bad name-he and the
fellow who despoils other people's property.
If the specimens finally admitted to your collection are
chosen with discrimination, the result will be a genuinely
valuable asset. Each of the kinds of materials described in
this book-minerals, gems, ores, crystals, rocks, meteorites
-can be assembled into a handsome, interesting, and
profitahle collection. Sands, pebbles, concretions, geodes,
stalactites and stalagmites, and many other forms are
also proper specialties around which to build a collection.
The list is ext nsive, making possible enough variety to
suit the taste of almost every person who has an interest
in the wonders of inorganic nature.
Fig. I4
Prospector's hammer and Pick
A mineral collector does not require elaborate equipment, but he may make or buy as much as he needs for
certain purposes. The one essential item in the field is a
prospector's hammer, with either a pick or a chisel opposite the hammer head. A sledge hammer, light or heavy,
will sometimes be worth taking along, especially if it
does not need to be carried far. A separate cold chisel is
also useful for wedging rock apart or freeing crystals from
crevices. Square-pointed and diamond-pointed steel tools
are additional equipment for extracting specimens. Excess rock may be trimmed away with the same sudden but
carefully planned blows used in the mining operation itself; a succession of light taps is usually as effective as one
hard blow and is more easily controlled.
Fig. I5
Collectors knapsack
Minerals, and especially crystals, should be well
wrapped in cotton or tissue paper and newspaper, together with a paper label giving adequate information as
to locality and identity, if known. Adhesive tape can be
used to attach a temporary label or identifying number.
The specimens should then be carried in a cloth, canvas,
or leather knapsack. Delicate crystals, particularly, ought
to be protected against the hazards of transportation,
which cause more damage to even the toughest rocks than
does anything else. Gloves to wear while working, a magnifying glass for examining small specimens, a notebook
to record your observations, a field guide like this one, a
camera, and a pocketknife are other desirable articles to
have. Inexpensive things such as an ice pick, a file, and a
ruler often come in handy. Adequate clothing should be
Fig. I6
Collectors pocket magnifying lem
..,.01..
... .1£!_
.$PHALIA' TE
,an.-t1
.5-r"L."C'-"T*'C
Mineral label
Fig. I7
Cou.c\or
a..&ru
rfl.£ 0
FOuwO
j .
H .... 0 ... "
AUG 3,
'Q 38
.... RHYoLITe
OCcu.rreDCI
Fig. I8
Rock label
COl1oc\or
_u
110.
POI1lCPHY~Y
5'0£
Oft
W.LLIA.M
....
~
1:, . .
But'tI-lS
[><CHA"G£O "011.
JASPCR.
worn, just as for any type of outing or camp life. Testing
apparatus is described later in this book, but identification is usually best made at your leisure at home or in a
laboratory.
Specimens should be trimmed carefully to size, but
without destroying the surrounding matrix of a crystal; a
pink topaz crystal in light-gray rhyolite from the Thomas
Range of U tab, for instance, is of more interest and brings
a higher price than the loose crystal, because it shows the
natural occurrence and associations. Another reason for
the greater value of a matrix specimen of topaz is that it
is less common than an isolated crystal. This is true of
nearly every mineral.
Good grooming will improve a mineral specimen. Most
specimens can be cleaned in soapy warm water applied
with a soft brush. Soluble minerals, such as halite (table
salt) , cannot, of course, be immersed in water, but alcohol will rinse them. A blast of air will remove dust from
fragile specimens such as the hairlike zeolites. A coating
of clear lacquer may aid in protecting specimens from soil
or tarnish. Each mineral has an individuality of its own,
and the technique of cleaning and preserving mineral
specimens may require some time to learn.
After cleaning them, you may give your specimens a
permanent label by writing or typing the data on adhesive
tape or lettering them in india ink on a patch of white
enamel painted on the surface. Most collectors like to
prepare a simple paper or cardboard label which will
stand or lie next to each specimen and show the essential
information about it. If this is done, the specimen itself
need bear only an accession number beginning with
1; a small label of this sort provides a neater-looking
specimen but makes necessary some kind of cataloguing
system. Therefore, the specimens are finally enumerated
and described in a notebook or card file. An index or
cross-reference list is desirable for finding quickly all
specimens of a given kind or from a given locality. The
sample labels on page 35 tell the least that should generally be known about a mineral or a rock.
Fig. I9 Mineral cabinet
with partitioned drawers
At first you may use simple storage boxes until your
collection expands and you feel the need of a good case to
set off some of your really beautiful specimens. M. F.
Wasson, a Denver attorney, has demonstrated an example
of the ingenuity possible in displaying minerals to the
best advantage, by illuminating an old china cabinet with
concealed strings of white Christmas-tree lights. Some
collectors who are wealthy or skilled in carpentry have
built-in wall cases in their homes or have bought standard metal floor cases of the kind seen in museums. Portable outfits resembling a traveling salesman's sample
case and consisting of removable sliding trays, which can
36
Fig.
Riker mount with glass top
20
be set on a table, are popular with collectors who own
them.
Gems and small crystals are easy to exhibit in Riker
mounts, which are frames that come in various dimensions; being paper-bound and glass-covered, they can be
handled without danger of damaging the contents and
can be suspended from a wall like pictures. Attractive
plastic boxes and more elaborate wood frames are coming
into the market, as interest in the mineral hobby expands.
Larger specimens can be mounted on individual stands of
metal, wood, glass, or plastic; besides being transparent,
plastic can be attractively lettered with a vibrating electric tool. There is no end to the possibilities in housing,
Fig.
2I
Plastic specimen mount
37
lighting, mounting, arranging, and labeling a mineral
collection for display.
Unless you are fortunate enough to be able to travel
extensively on every continent and collect all the specimens you want, you will sooner or later realize the benefit to your collection of well-selected purchases. Dealers
who advertise in the magazines named on page 187 have
world-wide connections, enabling you to have eventually
a thoroughly representative collection. Also, you will
learn faster by having at hand an inexpensive selection
of named samples to study and test. An increasing number of dealers maintain regular stores in which browsing
and shopping can conveniently be done.
Specimens that you do not find yourself or buy from a
dealer may be secured by exchanging with fellow collec·
tors; this is one of the benefits of membership in a club
devoted to the mineral hobby. Material to be traded
should be clean and well labeled, accompanied by a list
giving the pertinent information and particularly the
locality. Packages to be shipped by parcel post or express
should be well packed, for minerals often prove to be
disappointingly fragile.
Growing crystals artificially is becoming increasingly
popular. This is a creative activity which is in some ways
as satisfying as collecting in the field. It also has its scien·
tific aspects, for it helps us understand the conditions
existing in nature that govern the growth of minerals.
Copper sulfate, sodium chloride, potassium alum, bari·
um chloride, potassium ferrocyanide, and potassium fer·
Fig .
.22
Growing artificial crystal
ricyanide are among the chemical salts that will crystallize
readily, requiring only a saturated solution in a covered
flat dish, kept where the temperature is constant and outside interference is at a minimum.
As the solution slowly evaporates, the upper part becomes supersaturated with the dissolved salt. Being
denser, it sinks to the bottom of the dish, where it adds its
excess salt to the crystal that has already started to grow.
Perhaps a seed crystal of the same substance has been
placed in the dish for the purpose of encouraging growth.
Then the liquid rises again and the process continues
over and over, while the crystal increases in size. Larger
and more symmetrical crystals are made by suspending a
seed crystal on a wire to serve as a nucleus, though the
solution may need to be stirred from time to time.
The color of the crystal is often very pretty in itself,
such as the deep blue of copper sulfate, the lemon yellow
of potassium ferrocyanide, and the ruby red of potassium
ferricyanide. It can be varied by adding a small amount
of pigment, and wholly colorless crystals take on delicate
hues when a drop of ink or dye is put into the solution.
Changes in the shape of the crystal can be effected by
adding almost any foreign substance. Small crystals attach
themselves to the main one and may have to be removed,
after which the larger crystal heals over. A lifetime of
interesting experiments awaits the grower of artificial
crystals, with something new revealed every day.
Visits to museums are strongly encouraged as a means
of becoming familiar with choice specimens. Almost every
college and university has a geological museum, and
some cities boast truly magnificent exhibits. Among the
finest mineral collections on the North American continent are those in the American Museum of Natural History (New York), Harvard University (Cambridge,
Mass.), Philadelphia Academy of Natural Sciences,
United States National Museum (Washington), Royal
Ontario Museum of Mineralogy (Toronto), Chicago
Museum of Natural History, Denver Museum of Natural
History, and Los Angeles County Museum of History,
Science, and Art. In mining regions, especially in the
West, local material is often shown in the county courtS9
houses and in the windows of assayers. Before collecting
in a given locality, it is wise to look first at specimens of
minerals and rocks that have already been found there, in
order to be sure of their appearance and names.
At the end of the descriptive section you will find brief
notes about books especially recommended for further
reading. A small personal library is a source of much satisfaction to the collector as he grows in experience and
knowledge. These books contain information on many
subjects of interest to mineral and rock collectors, and
the locality guides furnish detailed instructions for reaching productive deposits in many states. These and more
detailed books can be borrowed through nearly every
public and school library, whether or not they actually
are on the shelves.
A list of the national magazines dealing with this hobby
is also given in this book. A good suggestion is that every
serious-minded collector, whether beginner or advanced,
should subscribe to one of these periodicals. The instruction and inspiration you will receive will more than
repay the small cost.
CHAPTER 4
Seven Keys to Recognizing Minerals
The secret of iden tifying minerals quickly and successfully is to become familiar with their common physical
properties. A property of a substance is any quality typical
of it, or anything pertaining to its appearance. Thus, a
sweet taste is a property of sugar, a white color is a property of snow, and heaviness is a property of lead.
Unless we can recognize a mineral by sight alone, it may
seem to be anyone of many minerals, but as soon as we
are able to determine one or two definite properties, we
have immediately reduced the long list of possible minerals to the names of a few probable ones. With only these
to choose from, we can much more easily decide the true
identity of the unknown specimen. The convenient arrangement of the Seven Keys used in this book ought to
enable you to recognize the most important minerals by
an easy step-by-step procedure.
MINERAL KEY NO. 1
LUSTER
Light is reflected from the surface of minerals in various
ways, producing a number of different types of luster.
First of all, every mineral has either a m etallic or a nOnm etallic luster, according to whether or not it resembles
the surface of a metal. This serves to put any unknown
mineral into one of two main groups without further
delay.
A metallic luster, though difficult to describe, is simple
to recognize, being the luster of a typical metal-gold.
silver, lead, copper, aluminum, and the rest. Minerals
having this luster are opaque, and when crushed they
yield a powder which is black or darker in color than
the mineral itself. (This powder is referred to as streak
and is described in Key NO.4.)
A nonmetallic luster is more complex, because several
kinds can be recognized. Minerals having a nonmetallic
41
luster become transparent on a thin edge, and when
crushed they give a powder which is white or lighter in
color than the solid mineral.
Perhaps the most common of the nonmetallic lusters is
vitreous, which means glassy; it is how ordinary glass
looks in reflected light. Quartz and many other minerals
have this sort of luster. Minerals such as mica that display
cleavage (described in Key NO.5) have a pearly luster,
which is due to closely parallel layers' trapping the light
and breaking it up into tiny rainbows. Diamond and non·
metallic lead minerals reflect a brilliant luster called ada·
mantine, which means diamondlike. Sphalerite is one of
the few minerals which shows a resinous luster, like that
of rosin from trees. A silky luster is displayed by fibrous
minerals such as gypsum of the satin-spar variety. Greasy,
oily, and other lusters may be described in common words
just as they appear to the observer.
MINERAL KEY NO. 2
HARDNESS
A rock is often called soft because the individual particles,
which may be hard in themselves, are loosely held together and readily fall apart. In speaking of minerals,
however, hardness means the resistance of the whole sur·
face to being scratched.
A century ago a mineralogist named Friedrich Mohs devised a scale of hardness which is still in use and known
as the M ohs scale. He placed talc, the softest of all min·
erals (soft enough to be made into talcum powder), as
NO.1 in this series, and diamond, the hardest of all known
substances, as No. 10. The complete scale is given below.
10
Diamond
9 Corundum
8 Topaz
7 Quartz
6 Feldspar
5 Apatite
4 Fluorite
3 Calcite
2 Gypsum
Talc
The Mohs scale does not indicate any exact hardness;
thus, No. 9 is not three times as hard as NO.3. It means
only that any mineral can scratch all those beneath it in
~
Rub scratch to distinqlAish
Use fresh
surfQceof'
unknown minerol
Fig.
from powder of'
harder minarQI
~3
Testing hardness
the scale and in tum will be scratched by those above it.
Two minerals of the same hardness will scratch each
other. For convenience, some familiar objects, which en·
able us to make tests quickly in the field, have been included in the scale, as follows:
2~
S
5~
6~
Fingernail
Copper coin
Knife blade, window glass
Steel file
Minerals under 2% will leave a mark on paper; those
under 5!/2 can be scratched by a knife; those over S!/2 will
scratch glass.
MINERAL KEY NO.
3
COLOR
Nature field guides customarily use color as a basis of
classification. This is generally true of those dealing with
birds, flowers, butterflies, etc., in which color plays a chief
part in identification. Minerals, however, are so variable
that we can scarcely rely upon color as a satisfactory
means of recognizing them.
Some minerals, it is true, are reasonably constant in
their color, such as yellow sulfur, pink rhodochrosite.
blue azurite, and green malachite. Most minerals, however, are colored by chemical elements that are really
4~
minor impurities and that produce a bewildering array of
colors without changing the essential composition. Quartz,
for instance, occurs in almost every imaginable color, as
do tourmaline, corundum, and numerous other minerals.
Nevertheless, certain colors are more or less characteristic
of particular minerals, and in the following outline and
descriptive section they are emphasized whenever they
may be useful in identifying the mineral.
You should note that many minerals have a tenden.CY
to tarnish somewhat, and so a freshly broken surface may
have to be exposed to reveal the true color. This is so
dominant a property of the mineral bornite, which
miners often call peacock ore and purple copper ore, that
bornite is listed here under the usual purple color of its
tarnish.
MINERAL KEY NO.
4
STREAK
Most minerals lose their color entirely when they are
finely crush~d. Some, however, still show a pale color,
similar to that of the whole mineral, but lighter. A few
Fig. 24
Testing streak of mineral on streak plate
mineral powders give a fairly vivid color, which may even
be rather different from the original one seen before
crushing.
The color of a powdered mineral is called its streak
because it is usually obtained by rubbing it against a
piece of unglazed porcelain. For this purpose ordinary
44
pieces of untreated tile are sold for a few cents as streak
plates, but the edge of a broken china cup or dish is adequate though not so convenient to handle. One mineral
for which streak is a useful property in identification is
hematite. No matter how black or steely it may look at
first, every piece of hematite gives an Indian-red streak;
the name of the mineral, which means "bloodstone," was
derived from this fact.
MINERAL KEY NO.5
CLEAVAGE
Crystalline minerals are said to cleave or have cleavage
when they break in definite directions along smooth surfaces. This interesting property is the result of a precise
pattern of atoms in regular layers, whose cohesion is
weaker in certain directions than in others. A piece of
mica will flake into thinner and thinner sheets because
it can be so readily cleaved in a single direction. Other
minerals have two, three, four, or as many as six directions
of cleavage.
Cubic
Octahedral
Fig. 25
Rhombic
Important kinds Of cleavage
Cleavage is often confused with an original crystal face,
though the pearly luster of the cleavage surface is a helpful clue. We describe cleavage according to how easily it
is obtained and what its crystal direction is. Diamond, for
example, has a "perfect octahedral" cleavage, which is
used by diamond cutters to remove flawed parts of a
crystal merely by splitting them away with a sharp blow.
45
D......
....
. :" ,
Cleavage IS a copstant enough property to serve as an
teexcellent means of dentifying minerals. It can best be
,_ recognized by sma 1 steplike surfaces on the outside in
preference to internal cracks, because these may be due to
- 'Other causes.
When a mineral breaks with smooth surfaces like those
of cleavage but only in certain favored places and on only
a few specimens, it is said to show pseudodeavage, or
parting. Because true cleavage and parting resemble each
other so closely, they are included together in the outline
and descriptive section of this book, though mentioned
separately under the individual minerals.
MINERAL KEY NO.
6
FRACTURE
Minerals that break in irregular directions, like shattered
glass, are said to fracture instead of cleave. The particular
kind of fractu~~ depends upon the appearance of the new
surface. When it has a series of arcs, typical of the growth
pattern of shells, it is known as conchoidal, meaning shelllike. Quartz and a host of other minerals have this con-
Fig. 26
Important kinds of fracture
choidal fracture, which is also seen on chipped glass.
Native metals such as copper and silver have a hackly
fracture, which gives a jagged surface uncomfortable to
touch. Other kinds of fracture are called simply even,
uneven, earthy, and so forth-ordinary words that describe their appearance.
46
MINERAL KEY NO.
7
SPECIFIC GRAVITY
Minerals vary considerably in weight, some being heavy
for their size, others light. The relative weight of a mineral
is known as its specific gravity, which is a number that
gives a direct comparison with the weight of an equal
volume of water. For instance, a specimen of corundum
having a specific gravity of 4.0 weighs four times as much
as the same volume of water.
Methods for determining specific gravity are beyond
the scope of this book, but experience in handling specimens will give you a surprising ability to estimate it
closely. A typical metallic mineral such as pyrite or hematite has a specific gravity of about 5. A typical nonmetallic
mineral has a specific gravity between 2.6 and 2.8, a range
which covers the common minerals quartz, feldspar, and
calcite. Minerals appreciably above or below these values
seem noticeably heavy or light when lifted. These average
figures are used as the basis of distinction in the outline
and descriptive section of this book. Thus, a specimen
which might be of medium weight for a metallic mineral
would perhaps be considered heavy for a nonmetallic
mineral-your own judgment and experience tell you
what to expect when you lift it.
OTHER MINERAL PROPERTIES
Minerals possess a wide range of physical properties besides those described above as the Seven Keys to Mineral
Recognition. Whenever any of these properties is typical
of a given mineral and is therefore a valuable clue for
identifying it, you will find the property described or
illustrated in the descriptive section of this book. The
more common and useful of these miscellaneous properties are discussed here now.
Many minerals belonging to the carbonate class will
effervesce, or fizz, in acid, a reaction involving the escape
of carbon dioxide gas. Household vinegar is sufficiently
strong. though you will often find it helpful to scratch
47
the specimen with a knife blade to form a powder which
reacts more quickly. Dilute hydrochloric (muriatic)
acid is the standard acid for this purpose. Of the minerals
described in this book, the following are carbonates: aragonite, azurite, calcite, cerussite, dolomite, magnesite,
malachite, rhodochrosite, siderite, smithsonite, strontianite. Some of these react more slowly than others. and
some satisfactorily only when the solution is warmed. The
only one that needs a different acid to dissolve it is cerussite which requires nitric acid to give the proper effect. If
anything stronger than vinegar is used. it should be handled carefully and carried in a bottle having an eye dropper or glass rod attached to the stopper for convenience in
touching it to the specimen.
Pyrite cube
~ Plagioclase feldspar
Fig. 27 Striations on crystals
Striations are parallel straight lines resembling a grating. They come about in several different ways. Some
mark the boundary between the parts of twin crystals,
where the twinning is repeated over and over again. This
sort of striation is seen on plagioclase feldspar. Others, as
shown on pyrite and quartz, result from a conflict between two kinds of crystal faces, each controlling the
growth for a while and one of them finally winning out.
Flexible minerals, such as chlorite, can be bent in your
fingers and will stay bent until forced in another direction. Elastic minerals, on the contrary, will snap back to
their original position unless they have actually been
48
broken; the mica minerals are typical examples of this
interesting property.
Malleable minerals are those which can be hammered
flat without falling to powder. The native metals-gold.
silver. copper-are malleable. though loosely held fragments sometimes crumble or come apart. Sectile minerals
can be cut with a knife. like horn or dry cheese. without
powdering. but they will pulverize if pounded with a
hammer. A few minerals described in this book. such as
gypsum. are somewhat sec tile.
Magnetic minerals are those which can be attracted by
a common horseshoe or bar magnet. After being heated.
or when tested with an electromagnet. hundreds of minerals are magnetic. but only two of the common minerals
-magnetite and pyrrhotite-can be picked up before such
treatment.
Fig. 28 Short-wave ultraviolet lamp
Fluorescent minerals glow in the dark when exposed to
invisible radiation. such as ultraviolet light. X rays. and
cathode rays. The most practicable source for such radiation is an ultraviolet lamp, which may be of two main
types. The long-wave lamp emits a good deal of heat. The
short-wave lamp. though more expensive. is cool and it
is effective with a larger number of minerals. Either one
causes colors to appear t.hat were not perceptible before.
A glass filter is employed to screen out the undesirable
visible light, but a dull purple almost always comes
49
through. Some of the most spectacular optical effects in
the mineral kingdom are obtained by fluorescence.
When the glow continues after the lamp is turned off,
the mineral is said to be phosphorescent. Other kinds of
so-called luminescence can be produced by heating or
striking certain minerals in the dark. Although fluorite is
the mineral which gave its name to the property, it does
not show fluorescence as well as some others. Of the ones
described in this book, scheelite and willemite can be
most relied upon to show fluorescence. Many diamonds
fluoresce, and so do many specimens of calcite and opal.
Battery-powered portable lamps have been a popular
item of equipment for prospecting for scheelite, an important ore of tungsten.
Fig. 29 Portable Geiger counter
Radioactive minerals break down spontaneously and
at a uniform rate. As the atoms disintegrate and change
to other elements, they give off energy. Although various
kinds of equipment will detect this radiation, the instrument most generally in use is the Geiger counter. By
means of a click, a flashing bulb, or a dial, the presence of
the invisible radiation is revealed. Portable models have
become common among prospectors, both professional
and amateur. Pitchblende (or uraninite) and carnotite
are the radioactive minerals described in this book, although zircon and a few others often possess this vital
property to a minor extent.
50
The ability of some minerals such as enargite and stibnite to fuse or melt in the flame of a match or candle is
worth remarking upon. Cryolite is unique among the
minerals included in this book in its property of becoming virtually invisible when immersed in water, because
there is very little difference in the light-bending power of
the mineral and the liquid.
Final identification of a difficult mineral might require
blowpipe tests, chemical tests or analyses, optical examination with a petrographic microscope, or X-ray photography. Such procedures are explained in the more advanced textbooks on mineralogy.
How to Read Chemical Formulas
The chemical composition of minerals is written in a special shorthand called a formula, which is really quite simple to read. It tells which chemical elements go to make
up the mineral and how many atoms there are of each
kind. For example, the formula of quartz is Si0 2 , denoting that this mineral is composed of silicon (Si) and
oxygen (0), the proportion being one atom of silicon
to two atoms of oxygen. Every chemical element is given
a symbol which is an abbreviation of either the English or
the Latin name. Thus, C stands for carbon, Ca for calcium, and Fe for ferrum , which means iron. The symbols
of forty of the most common elements occurring in minerals are included in the following list. Those marked
with an asterisk (.) are able to exist by themselves as
native elements or minerals in the earth's crust; more
than half of them are common or important enough to
be described in this book. The rest of the elements listed
below are combined with other elements in the form of
mineral compounds.
·Ag
AI
·As
·Au
B
Silver
Aluminum
Arsenic
Gold
Boron
Ba
Be
• Bi
·C
Ca
Barium
Beryllium
Bismuth
Carbon
Calcium
51
Ni
0
P
·S
Si
Chlorine
Chromium
Co Cobalt
·Cu Copper
Fluorine
F
• Fe Iron
Hydrogen
H
• Hg Mercury
Potassium
K
Lithium
Li
• Pb Lead
Mg Magnesium
Mn Manganese
Mo Molybdenum
Na Sodium
Cl
Cr
• Sb
Sn
Sr
Te
Ti
U
V
W
Zn
Zr
Nickel
Oxygen
Phosphorus
Sulfur
Silicon
Antimony
Tin
Strontium
Tellurium
Titanium
Uranium
Vanadium
Tungsten
Zinc
Zirconium
Certain radicals, or combinations of elements, are
especially frequent in minerals. These belong to various
major chemical classes, of which the most lmportant are
listed here, with an example of each, taken from this
book.
AI,O,
B,O" etc.
CO,
Cr,O.
MoO,
PO.
SO,
SiO"Si,O..
Si,O., etc.
TiO.
VO,
WO.
Aluminate
Borate
Carbonate
Chromite
Molybdate
Phosphate
Sulfate
Spinel
Borax
Calcite
Chromite
Wulfenite
Apatite
Barite
Silicate
Titanate
Vanadate
Tungstate
Zircon
llmenite
Vanadinite
Scheelite
Other chemical classes, consisting of a single element,
which combines with o~e or more different elements, include the following:
M
Cl
F
0,
S
52
Arsenide
Chloride
Fluoride
Oxide
Sulfide
Niccolite
Halite
Fluorite
Hematite
Galena
Water may be present in the form of H 20, as in gypsum, or as hydroxyl (OH) , as in topaz. The oxide of
silicon (SiO z ) is referred to simply as silica. When a
formula includes a comma, as (Fe,Al), it means that the
given atom may be of either element (here, iron or
aluminum, as in epidote). The letter n, as nH 2 0 in
opal, indicates a variable number of groups of atoms.
How to Use the Outlines
Through a step-by-step process of eliminating the classifications that do not apply to the specimen you are trying
to name, you ought to arrive at the section which includes
the right mineral. A few precautions, however, may prove
of value.
If you are not certain about the existence of anyone of
the properties upon which the outline is based, you will
be wise to examine either the opposite section or the
adjacent ones. For example, if the cleavage is probably
present but appears indistinct, try the sections headed
"Shows good cleavage" and "Does not show good cleavage." If the other properties are sure but the hardness is
doubtful, examine the sections varying in hardness from
the one you have almost decided upon. This technique
holds good for luster, color, and streak as well-of these,
the streak is most apt to cause uncertainty.
Each section begins with the minerals that are generally most easily recognized by their typical color or other
readily observed characteristics. This facilitates identifying the more difficult minerals, because once the simpler
ones are eliminated, attention can be confined to the
details of the less obvious ones.
A magnifying glass should be used to examine the
cleavage, striations, inclusions, or other features shown
in the drawings because they may be present on too small
a scale to be visible otherwise.
The color is indicated in the illustrations for those
minerals which belong to groups whose color is used as
a key for classifying them. For the rest the chief colors
are given in the scientific data above the illustrations.
53
Bornite is placed according to the purple color of its
tarnish, which is the hue almost always seen. The other
minerals are classified by the color of their fresh surface,
although the outside of the specimen is often somewhat
darkened or made iridescent by a film of tarnish. Hence a
newly broken surface should be exposed before deciding
the color, especially of a metallic mineral.
Identifying the Minerals. Outline of Keys
A. Metallic luster Page 56
1. Can mark paper Page 56
a. Blue color Page 56
b. Gray or black color Page 57
2. Cannot mark paper, but can be scratched by knife
blade Page 62
a. Copper color Page 62
b. Purple color Page 68
c. Red or brown color Page 69
d. Gray or black color Page 71
3. Can scratch glass Page 77
a. Brass color Page 77
b. Reddish·brown color Page 79
c. Gray or black color Page 80
B. Nonmetallic luster Page 85
1. Leaves colored mark on streak plate Page 85
a. Green or blue mineral color Page 85
b. Red or orange mineral color Page 88
c. Yellow mineral color Page 91
d. Brown mineral color Page 9.sJ
2. Leaves white mark or scratch on streak plate
Page 95
a. Shows good cleavage Page 95
1. Can be scratched by fingernail
Page 95
2. Cannot be scratched by fingernail, but can
be scratched by copper coin Page 99
54
3. Cannot be scratched by copper coin, but can
be scratched by knife blade Page 108
4. Can scratch glass and be scratched by qUattz
Page 124
5. Cannot be scratched by quartz Page 126
b. Does not show good cleavage Page 129
1. Can be scratched by fingernail
Page 129
2. Cannot be scratched by fingernail, but can
be scratched by copper coin Page 132
s. Cannot be scratched by copper coin, but can
be scratched by knife blade Page 136
4. Can scratch glass and be scratched by quartz
Page 146
5. Cannot be scratched by quartz Page 154
COVELLITE
KEYS: Metallic luster. Can mark paper. Blue color.
Streak: Gray or black. Cleavage: I direction.
Specific Gravity: 4.6-4.8 (medium weight).
Turns purple
when
wet-
Cleaves into
thin flexible
plates ·
Named after N. Covelli (1790-1829), the man who discovered it on Mount Vesuvius, covellite is copper sulfide
(CuS). It is not too common a m ineral, but a number of
good localities have been described, and it serves as a
minor ore of copper. It is associated with other copper
minerals, often in the richest part of the vein, and seems
to have altered from them. Most covellite does not occur
in crystals but rather in irregular pieces. Large crystals,
singly and in groups, beautifully surfaced with an iridescent coating, have come from the Calabona mine at AIghero on the Mediterranean island of Sardinia. Similar
specimens occur at Butte, Mont.-"the richest hill on
earth"-where covellite is fairly abundant. It is found on
Luzon Island in the Philippines and was formerly mined
at Kennecott, Alaska, in large m~sses of the distinctive
indigo-blue color that identifies this attractive metallic
mineral. Several places in Colorado and a few in Wyoming and Utah have furnished some covellite. Argentina
and other countries in South America also produce it.
Bor, Yugoslavia, and Leogang, Austria, are among the
chief European sources.
56
GA LENA
KEYS: Metallic luster. Can mark paper. Gray or black color.
Streak: Lead-gray. Cleavage: 3 directions.
Specific Gravity: 7.4-7.6 (heavy).
lead gray. shiny
DIAl! if cooted
-4---+-- Cubic
cleavClge
Galena is lead sulfide (PbS). It is by far the chief ore of
lead. as well as an important ore of silver. which is present
as a valuable "impurity." Silver is especially apt to be
found by assaying the fine-grained variety known as steel
galena, in which the right-angled cleavage that is so characteristic 01 the mineral may not be readily visible. Worldwide in distribution, galena is noteworthy in places as
separated as Broken Hill, Australia; Freiberg and the
Harz Mountains, Germany; the mining districts of CornwaU, Cumberland, and Derbyshire. England; and Leadville, Colo. In the United States galena can be found in
metal-bearing veins and in limestone at many localities. It
is particularly abundant in the Mississippi Valley from
Wisconsin to Missouri, in the Tri-state area where Kansas.
Oklahoma, and Missouri come together, and in the Rocky
Mountain states of Idaho. Utah, and Colorado. The
otherwise shiny surface of galena becomes dull when
coated with a film of other substances; and. upon oxidizing, galena alters to anglesite and cerussite. Interesting
combinations of galena with other minerals are often
found in various rocks. Such an occurrence is the one at
the Sullivan mine, Kimberley, B. C.
57
STIBNITE
KEYS: Metallic luster. Can mark paper. Gray or black color.
Streak: Lead-gray. Cleavage: I direction.
Specific Gravity: 4.6 (medium weight).
Cleavage along len9th
Curved crystal
lead gray
Often tarn '
Antimony sulfide in chemical composition (Sb 2Sa). stibnite is the most important ore of antimony. From the
extensive mines at Ichinokawa on the island of Shikoku.
Japan. have come truly magnificent crystals. nearly 2 feet
long and possessing many brilliant terminal taces. Groups
of these are in the proud ownership of leading museums.
An unusual feature of stibnite crystals is their curved or
twisted appearance. as though they had been warped. The
excellent crystals from Felsobanya. Rumania, sometimes
penetrate tabular specimens of barite. At Bau. in Borneo.
large crystals are also found. Some well-crystalhzed material occurs near Hollister and elsewhere in California.
Massive stibnite is produced in quantity in Hunan Provo
ince, China, where the world's largest deposits are situated. Stibnite is fusible enough to be melted in a match
flame. White and yellow oxides of antimony often coat
its surface, and stibnite loses its brightness when exposed
to light. The dark powder was used by the ancients to
blacken their eyebrows. Today stibnite enters directly
into the manufacture of fireworks, vulcanized rubber,
and medicines.
58
MOL YBDENITE
KEYS: Metallic luster. Can mark paper. Gray or black color.
Streak: Dork-groy, with greenish tinge.
Cleavage: I direction.
Specific Gravity: 4.6-4.7 (medium weight).
Seetile
A sulfide of molybdenum (MoS 2 ) and the principal ore
of that strategic industrial and military metal, molybdenite is so tricky to pronounce that miners simply call it
"molly." Crisscrossed throughout with thin veins of it is
Bartlett Mountain, rising behind Climax, Colo., to an
altitude of 11,300 feet-the greatest concentration of
molybdenite on this planet. For large crystals, however,
the collector would have to go to such places as Bluehill
Bay, Me. ; Edison, N.].; Renfrew County, Ont.; or as distant as Kingsgate, New South Wales. Thick sheets of the
mineral have been taken from the very old rocks that
underlie New York City. The resemblance between
molybdenite and graphite is strong; perhaps the bluish
tone to its color and the greenish tinge to its streak on
porcelain will help you recognize molybdenite, and also
it is heavier. Molybdenite is commonly found in granite
and quartz, which are not the typical host rocks for graph.
ite. The yellow dust on the top and in cracks is an altera·
tion product. ferrimolybdite. which might easily be mistaken for sulfur or one of the secondary uranium minerals.
59
GRAPHITE
KEYS: Metallic luster. Can mark paper. Gray or black color.
Streak: Block. Cleavage: I direction.
Specific Gravity: 2.1-2.2 (light).
------/Black to gray
Scaly
cleavage
Flexible
~ales
Like diamond, graphite consists solely of crystallized carbon (C). The extraordinary difference between these two
minerals in color, hardness, conductivity, and crystallization is the most remarkable contrast in all mineralogy.
Whereas diamond is a nonconductor of electricity, graphite is a good conductor; whereas diamond crystals are
common, graphite crystals are scarce and poorly developed. A curious fact is that graphite is really the more
stable form of carbon, for diamond will change to it when
heated. \The greasy feel of graphite and the way it soils
the fingers indicate its uses as a lubricant for machinery,
a protective coating for metals, and the so-called lead for
pencils. This last use, its original one, gave graphite its
name, from the Greek word for "to write." Fine crystals come from Sterling Hill, N.J. The best-known occurrence in the United States is near the Revolutionary War
fort of Ticonderoga, N.Y. Graphite is also found with
coal in Rhode Island. Korea, however, has produced more
commercial graphite in one year than any other country.
A more consistent output comes from Madagascar, Ceylon, Mexico, Siberia, and Central Europe.
60
PYROLUSITE
Keys: Metallic luster. Can mark pa per. Gray or black color.
Streak: Black. C leavage: 2 directions.
Specific Gravit y: 4.4-4.5 (medium weight).
Black- - -..
cleava ges
Splin+ery --~
Radiating fibers
An oxide of manganese (Mn0 2 ) and the most abundant
ore of that metal, pyrolusite is a widespread mineral.
When in well·developed crystals, such as those of Horni
Blatna, Czechoslovakia, it is termed polianite, which is
very different from the common massive pyrolusite, being
hard enough then to scratch glass. Most frequently the
crystals are actually those of another manganese mineral
called manganite, which ha"e changed chemically, losing
water but retaining their original shape. Pyrolusite is at
its charming best when it assumes fanciful scenic patterns
in chalcedony quartz, turning it into moss agate. Similar
plantlike designs are developed on slabs of sandstone and
limestone. Ordinary pyrolusite, on the contrary, which
is often soft enough to soil the hands a dirty black, is
scarcely attractive. Sometimes it takes on a characteristic
bluish cast. Usually pyrolusite is mixed with other manganese ores, all of them being mined together under the
name wad. The most extensive deposits are in the Soviet
Union, Central India, Union of South Africa, Gold Coast,
and Brazil. Pyrolusite acts as a decolorizing agent in glass
and is used in batteries.
GOLD
KEYS: Metallic luster. C annot mark paper, but can b.
scratched by knife blade. Copper color.
Fracture: Hackly.
Specific Gravity: 15.6-19.3 (very heavy).
Native gold (Au) is never found entirely pure. A varying
proportion of silver is always present; when the percentage is high, the alloy is called electrum. Gold from Australia is noted for its low silver content. Well-developed
crystals are extremely rare, making their appealing
beauty all the more tantalizing. Exquisite, indeed, are
the dendritic or fernlike groups. In public museums in
Denver and nearby Golden, Colo., are displayed two
vaults of wonderful specimens from that state. Gold
ranges in size from the Welcome Stranger nugget found
at Ballarat, Australia, which weighed 2,280 ounces, down
to the "flour gold" in the Snake River, Ore., which requires about 5,000 of the minute particles to be worth one
cent. Gold can be distinguished from the various kinds of
"fool's gold"-pyrite, chalcopyrite, mica-by its heaviness
and malleability. Since 1886 the South African district of
Witwatersrand, familiarly called the Rand, has produced
over 16,0 00 tons of gold valued at more than 12 billion
dollars, from dozens of mines centering around J ohan nesburg. T he ranking gold mine of the Western Hemisphere
is the H omestake mine at Lead, S.D.
61
COPPER
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Copper color.
Fracture: Hockly. Specific Gravity: 8.9 (heavy).
Often green or blue
Copper red -----::i>Tarnishes LI,,,,,-,,'--r
Molleable--..
Native copper (Cu) was formerly the main source of copper. but various sulfide minerals now provide most of our
needs for this indispensable metal. The native mineral is
still a favorite of collectors. however. because of the interesting shapes it assumes. The stupendous deposits on the
Keweenaw Peninsula in northern Michigan dwarf everything else of their kind in existence. Here, for more than
a mile in depth, native copper, from great masses to tiny
specks. has been mined. Branching skeletons, tiny scales,
twisted wires, hollow shells shaped like human skulls,
stream-worn pebbles-these are all typical of the Michigan copper. One enormous piece weighing 420 tons was
found in 1857. Combinations of copper and native silver
are called "half-breeds"; both minerals soon become tarnished and look alike, but when cleaned they stand in
striking contrast to each other. Another Michigan occurrence consists of copper enclosed in transparent calcite; unaffected by tarnish, the copper gleams in its pristine state like gold. Native copper is also found at
Corocoro, Bolivia, and in small amounts in other countries. The island of Cyprus, an early source of copper,
gave its name to it.
Minerals in. Color
The following pages contain illustrations 01 £OIt}-six
minerals in lull color. Arranged in alphabetical order for
easy reference, they will be a further aid in identif)irtg
some of the minerals that are de~(rib ed in the text. All of
these colored illmtrations are reproduced by courtesy of
\Vard's Natural Science E~tabli~hment , Inc., Rochester,
Ne\\' York. The) al e from Ward's ColO! Slides [or Mineralogy (photographs by Katherine H. Jensen ), and from
the Har\'ard Mineral Collection.
AMBER
ANDAlUS'TE
Bailie.
\'ariel} Chia~LOlile.
Madera Count} , California.
APATITE
AURICHAlCITE
Auburn, Maine.
Kell >. :-.Ie" Mexico.
BERYL
AZURITE
Variety Aq uamari ne. Klein
Spitlkopje, South -West Africa.
Bisbee. Ari/olla.
BARITE
CALCITE
FelsObanya, R umania.
Egremont, Cllmberland,
England.
CHALCOPYRITE
St. Agnes, Cornwall, England.
CINNABAR
!\Iel'Lllr, t ' ta ll.
•
COPPER
CROCOITE
Hough lon Counly, Michigan.
:-lear Dundas, Tasmania.
CUPRITE
DATOllTE
Va riety Cha!cotrichite,
Bisbee, Ari7.0na.
Houghton County, Michigan.
DIAMOND
FLUORITE
Kimberley, South Africa.
Cave·ill · Rock, Illinois.
FRANKLINITE
GOETH ITE
Franklin, New ) C ISC).
hhpemi "g•.\ 1it;higan.
GOLD
GYPSUM
Cederherg Mine,
E I Dorado County, Calirol n ia.
Variety Selenite.
Elsworth, Ohio.
CARNOTITE
HALITE
Wjc)jczka, (,aJicia.
.)~Il
JlIan
COllnt).
lolorado.
HEMATITE
:'>[iIl3' Gcrai\. Briltit.
IRON METEORITES
LIMONITE
l\cl' \-' <'1)\111\). l' clIlI') halll'1.
MALACHITE
Bishee. A ri/on a.
MICROCLINE
, 'ariel > '\rnaLOll~JOl)c.
ne;lr Florbsa n l. Colorado.
MIMETITE
\ 'aricly Camp)/i(c.
n r~ Gill, CUlllherland. England.
Ilcnhlll\.
\ll~ll,dia,
ORPIM~NT
PYRITE
MerclIr, l·tah.
l\il1~hatll Cal1~on ,
QUARTZ
QUARTZ
" ariel> Jasper,
Otegoll.
" a ricl\ Roc\.. Cr"lal.
I.iu'e , "lis, 'e", \011-.
RHODOCHROSITE
RHODONITE
"ariel' Fo" lerite,
Uockenrod, Odenwald,
( , ermal1).
l ' lah.
rran\..lin. :-"cw JCI,e~.
SILVER
SMITHSONITE
H ou)!;htol\ (".oUllt}.
Kell ),
l\C\\
:\(c\.i(;(l.
\1 ithigan.
STlBNITE
SULFUR
lchinol..awa I }o.
Shikoku Japan.
Ciancian3. Sicily.
TOPAZ
•
De\ ils Head, Culonldo.
TOURMALINE
\( adaV;<\~car •
CHALCOPYRITE
KEYS: Metallic luster. Ca nnot mark paper, but can be
scratched by knife blade. Copper color.
Strea k: Greenish-black. Fracture: Uneven.
Specific Gravity: 4.1-4.3 (medi um weight).
A copper-iron sulfide (CuFeS 2 ) , chalcopyrite was so
named in 1725 to distinguish it from regular pyrite,
which does not contain copper except as an impurity.
Like pyrite, it is one of the minerals referred to as fool's
gold, and its brassy color resembles gold more than that
of pyrite does. Chalcopyrite may also tarnish to give an
iridescent appearance. Unlike pyrite it can be scratched
by a knife, and unlike true gold it is brittle. It is not only
the most common of all copper minerals, but it also frequently carries valuable amounts of gold or silver. It is
associated with nickel deposits in a number of places.
Rich pieces occur in the Noranda gold mine in the Rouyn
district of Quebec. The mines at Ducktown, Tenn.,
should be mentioned {or the quality of their chalcopyrite.
Large perfect and sharp crystals have been obtained at
Ellenville, N.Y., and in Chester County, Pa. A curious
massive type, found in some mines in Cornwall, England,
has a rounded surface and is known as blistered copper
ore. Distributed throughout a huge ore body at Rio
Tinto, Spain, chalcopyrite has long been mined there for
copper and gold.
NICCOLITE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Copper color.
Streak: Brownish-black. Fracture: Uneven.
Specific Gravity: 7.8 (heavy).
ColumnaI'
structure
A nickel arsenide (NiAs), niccolite is unusually attractive for a metallic mineral. Many times called copper
nickel by miners because of its coppery color and nickel
content, it is one of the lesser ores of nickel. As early as
1694 it was known in Germany as Kupternickel, and from
this name was taken our word for the white metal. Concentric shells a foot in diameter, alternating with zones
of arsenopyrite, come from Natsume, Japan. Crystals are
exceedingly rare, a few having been found at Richelsdorf
and Eisleben, Germany. The typical occurrence of niccolite is with silver, cobalt, and other nickel ores. These
associations are important in Germany, Czechoslovakia,
and Canada. In the United States some niccolite has been
reported from New Jersey and Colorado. A thin green
crust of annabergite often covers specimens of niccolite
and may be a useful clue for determining the presence
of niccolite beneath the ground. The reddish tinge of niccolite is different from the color of any of the other
minerals described in this book, though it is similar to
that of certain copper arsenides, and so a test for nickel
might be necessary.
66
PYRRHOTITE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Copper color.
Streak: Dark greyish-bleck. Fracture: Uneven.
Specific Gravity: 4.6 (medium weight).
Granular
texture
---~~~
Pyrrhotite is iron sulfide (FeS), with always a slight deficiency of iron that puzzled mineralogists for a long while.
It has been called magnetic pyrite because it somewhat
resembles pyrite and it is the only common mineral except magnetite to be noticeably magnetic. Thick crystals
have been collected at the Morro Velho gold mine in
Brazil. Smaller but good crystals have come from Kongsberg, Norway, and Andreasberg, Germany. Large crystal
groups are known at Loben, Austria, and elsewhere in
Central Europe. Great amounts of massive material are
mined at Sudbury, Ont., where they are intimately mixed
with pentlandite, a nickel sulfide that resembles pyrrhotite but is not magnetic. Ducktown, Tenn., is an important locality for ordinary pyrrhotite. The Western states
yield this mineral in a moderate quantity. The word
pyrrhotite, with its so difficult spelling, means reddish.
The variety called troilite has been known in meteorites
since 1766 and comes close to being pure iron sulfide.
Troilite of terrestrial origin has been found in Del Norte
County, Calif.
BORNITE
KEYS: Metallic luster. Cannot mark paper, but cit" be
scratched by knife blade. Purple color.
Streak: Gray-black. Fracture: Uneven.
Specific Gravity: 5.1 (medium weight).
This copper-iron sulfide (Cu 5 FeS.) is a significant ore of
copper. Although bornite is a common enough mineral,
crystals of it are rare, and so only irregular masses are
usually found. Even these pieces, which should be readily
recognized by their brownish-bronzy color on a freshly
fractured surface, are disguised by the purplish tarnish
which steals over them, covering them with a bright
iridescence and giving bornite the miners' names of peacock ore, purple copper ore, and horseflesh ore. Any specimen suspected of being bornite should be broken open to
expose a new area for observation, to see if the tarnish
gives way to a bronze hue. The color of the bornite from
Androka, Madagascar, is very choice. Large amounts of
bornite are mined in a number of countries of Latin
America and at Butte, Mont. It is also important at Mt.
Lyell, Tasmania. The rare crystals used to be found in
the United States at Bristol, Conn. This mineral was
named in 1832 after Ignaz von Born, an eminent Austrian
mineralogist who lived from 1742 to 1791.
68
CUPRITE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Red or brown color.
Streak: Brownish-red . Fracture: Uneven.
Specific Gravity: 6.0 (medium weight).
Native copper
present
! 4 - - - often
Copper oxide (Cu 2 0) in composition, cuprite is an ore of
copper; its name refers to this metal. The crystal forms
of cuprite are highly interesting to the mineral collector.
The most fascinating of the forms consists of a brilliant
red mat of haiTtike crystals, drawn out to needte tength;
this variety is called chalcotrichite (meaning "haiT copper") and is much in demand for its exceptional beauty.
The best specimens come from Morenci, Ariz., and from
old copper mines near Gwennap, England. Fine crystals
of the usual types of cuprite occur in several mines in
Cornwall, England, and even larger ones come from
Chessy, France. The name ruby copper suggests the appearance of the more desirable specimens. At Gumeschevsk in the Ural Mountains pieces of cuprite are perforated by cavities lined with later crystals of the same
mineral. Cuprite is an important source of copper at
Bisbee, Ariz. It is associated with other copper minerals,
and some native copper is almost always present and aids
in its identification. Ancient bronze and copper objects
of art show tiny crystals of cuprite which have grown
upon them during the passage of centuries.
GOETHITE
KEYS: Metallic luster. Cannot mark paper, but can
scratched by knife blade. Red or brown color.
Streak: Brownish-yellow. Cleavage: I direction.
Specific Gravity: 3.3-4.3 (medium weight).
b.
Iron hydroxide with the chemical formula HFe0 2 was
named goethite after the German poet and philosopher
Johann Wolfgang von Goethe (1749-1832), who was a
student and collector of minerals. It has undergone the
strange experience of a mineral which was one day believed to be an uncommon one and almost the next day
was learned to be one of the most abundant minerals.
This odd development was the result of X-ray examina·
tions which revealed that most of the iron ore that had
always been known as limonite was actually crystalline in
its atomic structure and therefore should really be called
goethite instead. Fine crystallized specimens are found in
cavities in granite in the Pikes Peak region of Colorado.
The large cubes of goethite found at Pelican Point on
the shore of Great Salt Lake, Utah, were originally pyrite
and have changed chemically. Lostwithiel, England, is a
classic locality for splendid crystals of goethite. This mineral is the main constituent of the important iron ores of
Alsace-Lorraine, and it is widely distributed in Cuba.
Goethite is found among the other iron minerals of the
Lake Superior deposits, especially at Negaunee, Mich.
'10
SILVER
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Gray or black color.
Streak: Silver-white. Fracture: Hackly.
Specific Gravity: 10.5 (very heavy).
When free of tarnish, silver (symbol, Ag) can be a handsome mineral. With its fernlike designs, network patterns, and crystal groups it resembles copper and gold
except in color. Wire silver and crystals of unsurpassed
beauty were produced for hundreds of years at Kongsberg, Norway, some of the masses weighing as much as
750 pounds. At Freiberg and Schnee berg, Germany, are
situated other silver mines of considerable age. Aspen,
Colo., is famous for its twisted masses of wire silver, and
for the largest silver nuggets ever found, one of which.
weighing 1,840 pounds, was too large to be hauled from
the Mollie Gibson mine in an ore bucket. The fabulous
"silver sidewalk" of the La Rose mine at Cobalt, Ont.,
was a slab of almost solid metal 100 feet long and 60 feet
thick. Another Canadian silver locality is in the pitchblende deposits at Great Bear Lake. The Coeur d'Alene
district in Idaho is also a prominent place for native silver. Mexico is the world's leading producer of silver,
which may well be regarded as a Western Hemisphere
metal, even though known to prehistoric man on the
other continents.
ENARGITE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Gray or black color.
Streak: Groyish-black. Cleavage: 2 directions.
Specific Gravity: 4.4 (medium weight).
Two
cleavages
A sulfide of copper and arsenic (Cu aAsS 4 ) , enargite is an
ore of copper, closely resembling stibnite except for its
somewhat darker color-a test for copper is the easiest
way to distinguish between them with certainty. Enargite
is particularly important at Butte, Mont., where it also
yields arsenic oxide ("white arsenic") for the chemical
industry. The deposits in Utah at Bingham Canyon and
Tintic also produce substantial amounts of enargite. California, Nevada. and Colorado furnish small quantities,
especially the silver mines in the San Juan Mountains of
Colorado. Outside of the United States the main sources
of enargite are in South America, in the noted copper
mines at Cerro de Pasco and Morococha. both in Peru,
and at Chuquicamata, Chile. A large deposit exists at
Bor, Yugoslavia, but otherwise enargite is not common
in Europe. The name of this mineral comes from a Greek
word meaning distinct, because of its definite cleavage,
and was applied to it in 1850. As antimony replaces the
arsenic, enargite grades into famatinite, which was named
after the Sierra de Famatina, Argentina.
7~
PYRARGYRITE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Gray or black color.
Streak: Brownish-red. Cleavage: 3 directions.
Specific Gravity: 5.8 (medium weight).
Gray---+
Ruby red on
thin l2aCU2--+~
Pyrargyrite, sulhde of silver and antimony (AgsSbS s), is
called dark-ruby silver because of its deep ruby·red color.
As atoms of arsenic substitute for those of antimony,
pyrargyrite grades into proustite, known as light-ruby
silver. Except for the transparency and depth of color,
these two minerals closely resemble each other, and the
two together are called the ruby silvers. Choice crystals
of either are clear and richly colored, like fine garnets or
rubies. Most ore specimens, however, are gray until
freshly broken, when the reddish hue appears on the
newly revealed face like a blush. Pyrargyrite is the more
abundant of these two minerals and is an important ore
of silver, occurring in many mines in tbe Western states
and South America. Exceptional crystals of pyrargyrite
come from Andreasberg, Germany, and magnificent ones
of proustite-among the most beautiful minerals known
-from Chanarcillo, Chile. In 1865 an enormous mass of
c:rystalline proustite weighing over 500 pounds was taken
from the Poorman mine at Silver City, Idaho, which has
also yielded substantial amounts of pyrargyrite.
75
PITCHBLENDE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Gray or black color.
Streak: Brownish-black. Fracture: Uneven.
Specific Gravity: 6.5--8.5 (heavy).
Geiger coonter clicks ~
The principal material for the creation of atomic energy
and the primary source of uranium, pitchblende is today
without doubt the world's most strategic mineral. It is
also one of the most historic of minerals, having been the
original source of radium and the first substance in which
helium (earlier detected in the sun) was discovered on
the earth. Pitchblende is uranium oxide with a mixture
of products caused by weathering and alteration. When
the same mineral is of purer composition (dominantly
U0 2) and occurs in crystals, it is called uraninite. The
ratio between the uranium and the lead or helium formed
by radioactive disintegration is a measure of the time
that has elapsed since the mineral crystallized. The name
pitchblende refers to the pitchy luster. The Katanga district of the Belgian Congo, and Great Bear Lake, Canada,
are the main producers of pitchblende and uraninite.
Older deposits of especial prominence are situated at
Sankt Joachimsthal (now Jachymov), Czechoslovakia.
and Central City, Colo. Sparsely scattered crystals are
collected at Grafton Center, N .H" and several places in
Connecticut.
74
rETRAHEDRITE
KEYS : Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Gray or black color.
Streak: Black or brown. Fracture: Uneven.
Specific Gravity: 4.6-5. 1 (m ed ium weight).
Gray
Tetrahedrite is a sulfide of copper, iron, and antimony
[ (Cu,Fe) 12Sb,SlS]' together with several other metals, of
which silver is the most valuable. Hence it has a complex
composition, and it serves as an ore of both copper and
silver. First-rate crystals occur at Bingham, Utah; B6tes,
Rumania; Ste.-Marie-aux-Mines, France; and Liskeard,
England. The name of the mineral comes from the tetrahedral shape of the crystals-a wedge-shaped pyramid.
Tetrahedrite is well distributed among the mining districts of Idaho, Utah, New Mexico, Nevada, Arizona, and
California. Roman coins thrown into a hot spring at the
French watering place of Bourbonne-les-Bains have been
recovered with a natural incrustration of tetrahedrite. As
the content of antimony decreases while that of arsenic
increases, tetrahedrite grades into tennantite, named in
honor of an English chemist, Smithson Tennant (17611815) . Both minerals look alike; probably a good share
of the Colorado specimens labeled tetrahedrite are actually the other member of this series. Definite and complex
crystals of silver-bearing tennantite come from the Bin.n en tal, Switzerland.
75
CHALCOCITE
KEYS: Metallic luster. Cannot mark paper, but can be
scratched by knife blade. Gray or black color.
Streak: Gray. Fracture: Conchoid,,1.
Specific Gravity: 5.5-5.8 (medium weight).
lead gray,
shiny---+
Chalcocite is copper sui6de (Cu 2 S), one of the major
ores of copper. In the type of deposit referred to as "porphyry copper" it occurs sparsely disseminated through
the rock as a low-grade ore, which, however, can be extracted cheaply on a large scale. A zone spoken of as the
"chalcocite blanket" is a feature of such deposits, which
include Bingham in Utah and Ely in Nevada. Other important localities for chalcocite are Butte, Mont., and
Kennecott, Alaska. Pure masses weighing tens of thousands of tons are common at Kennecott, where the most
interesting occurrence is the "glacial ore," surrounded by
ice. Large amounts of chalcocite also exist at Tsumeb,
South-West Africa. Although scarce, choice crystals have
come from Bristol, Conn., and mines in Cornwall, England. Somewhat sectile, chalcocite should not be confused
with argentite, which can be cut much more readily. Like
other copper minerals, chalcocite often has a thin coating
of blue or green carbonate. Its shiny luster is seen only
on fresh surfaces, which rather soon turn dull and black.
The name comes from the Greek language and indicates
brass or copper.
76
PYRITE
KEYS: Metallic luster. Can scratch glass. Brass color.
Streak: Gree nish- to brownish-black. Fracture: Uneven.
Specific Gravity: 5.0-5.1 (medium weight).
The most familiar form of iron sulfide (FeS2) is pyrite.
Its name alludes to the sparks that fiy when it is struck
with a hammer. More abundant than marcasite, which
has the same chemical composition, it ranks as one of the
commonest of minerals. Pyrite, in fact, is referred to as a
persistent mineral because it occurs under almost every
geologic condition. It is one of the minerals called fool's
gold. Huge crystals of pyrite have come, among other
places, from Leadville, Colo.; Ubina, Bolivia; and the
island of Elba. The well-shaped crystals from Chester,
Vt., are attractively embedded in a green rock. So-called
iron-cross twins come from Schoharie, N.Y. At Sparta, Ill.,
are the "pyrite suns," which are radiating disks of the
mineral. Of little value for its iron content, pyrite is often
rich in gold and copper, as well as being a source of sulfur. Very large deposits occur at Rio Tinto, Spain, where
they have been worked since the days of the Phoenicians.
Some of the most interesting occurrences of pyrite are
those in which it has replaced the organic matter of fossils; a few localities for such specimens are Holland
Paten t, N.Y., and Lyme Regis, England.
77
MARCASITE
KEYS: Metallic luster. ell" scrlltch glllss. Brllss color.
Streak: Greenish-black. Fracture: Uneven.
Specific Gravity: 4.9 (medium weight).
Having exactly the same composition as pyrite, though
not nearly so common, marcasite is iron sulfide (FeS z).
The curious spear and cockscomb forms of marcasite are
actually twin crystals; at Dover, England, the spearshaped specimens are believed locally to be relics of
Roman weapons. Marcasite often grows as stalactites covered with pyramid-shaped crystals projecting outward
like spikes on a war club. Rounded lumps of various sizes
occur embedded in shale. In the former state of Wiirttemberg, Germany, fossils from the age of dinosaurs are
found to be replaced by marcasite. Leading localities for
marcasite in the United States include Mineral Point,
Wis., Galena, Ill., and the zinc and lead mines around
Joplin, Mo. The name marcasite used to be applied to
pyrite untjl a distinction was made between the two minerals, which have different atomic structures. Marcasite is
much less apt than pyrite to be present as individual
crystals, tending instead toward fibrous and radiating aggregates, especially in nodules. Because marcasite decomposes so readily, specimens fall to pieces quickly; unless
they are kept in a dry atmosphere, they become covered
with white powder.
78
HEMATITE
KEYS : Metallic luster. C a n scratch glass. Reddish-brown
color.
Streak: Reddish-brown. Fracture: Uneven.
Specific Gravity: 4.9-5.3 (medium weight).
The word hematite, which refers to the iron oxide mineral having the formula Fe 2 0 S ' means bloodstone. It is
certainly appropriate enough, on account of the Indianred streak of this vital iron ore. There are several distinct varieties of hematite. When it occurs in crystals, it
is black and shiny; Cumberland, England, and the island
of Elba are the most noted s urces for these handsome
specimens, which are cut into intaglios (reverse cameos)
for men's rings. Specimens that look like mica consist of
small flakes which shine like mirrors-this kind is called
specularite or specular hematite. So-called "iron roses,"
daintily curved like the petals of a flower, come from the
St. Gotthard area, Swiuerland. Rounded lumps are
called kidney ore, and these are especially fine at Ulverston, England. Commercial deposits of hematite are of
incredible extent in the Lake Superior region, along the
Appalachian Mountains, in eastern Canada, and at Cerro
Bolivar, Venezuela. Some of this material, called pencil
ore, is fibrous; some of it, called oolitic hematite, consists
of small rou nded bits that resemble fish roe.
79
MAGNETITE
KEYS: Metallic luster. Can scratch glass. Gray or black
color.
Streak: Black. Fracture: Even.
Specific Gravity: 5.2 (medium weight).
Octahedral
~~~
Owing to its strong magnetism this iron oxide mineral
(Fe s 0 4 ) has been known since early times. Not only will
a magnet pick up pieces of it quickly, but some speci.
mens act as natural magnets themselves-these are called
lodestone. An old story names magnetite after a shepherd,
Magnes, whose shoes and staff were held to the ground
by the attraction of lodestone for the iron nails in them.
Powerful lodestone. exhibiting a north and a south pole,
is found in the Ural, Altai, and Harz Mountains and on
Elba. It also occurs, together with crystals of magnetite,
at Magnet Cove, Ark. Other localities for excellent crystals are at Nordmark, Sweden; Arendal, Norway; Greiner, Austria; Scalotta, Italy; Vascau, Rumania; and Fiormeza, Cuba. Magnetite is the richest and one of the most
abundant ores of iron, for which it is mined on a vast
scale in Sweden, the Adirondack Mountains in New
York, and the so-called Bushveld Complex in South
Africa. Most of the black sand seen on ocean beaches,
shifted into streaks by the waves and currents, is composed of magnetite grains washed out of rocks in the
interior of the continent.
80
WOLFRAMITE
KEYS: Metallic luster. Can scratch glass. Gray or black
color.
Streak: Brown or block. Cleavage: direction.
Specific Gravity: 7. 1-7.5 (heovy).
An iron-manganese tungstate [(Fe,Mn) WO.], wolframite is the most important ore of tungsten or wolfram,
furnishing haH of the world's supply from deposits in
China. The richest source of all is in the Nan Ling
Range in the southern part of China. Large reserves are
also known in Burma, Malaya, Australia, and Bolivia.
Fine crystals come from CzedlOsJovakia, Germany, and
England. Other good localities for wolframite are in
Portugal, Spain, the Transvaal, and Siberia. Wolframite
is often associated with cassiterite, the chief ore of tin. In
the United States it has been found to some extent in
Arizona and the Black Hills of South Dakota. Wolframite
has had different names in several languages, each derived from the same odd Latin name "wolf froth," the
meaning of which is quite obscure. With a reduction in
its proportion of iron, wolframite grades into huebnerite,
which is very similar in appearance though bi-owner, nd
less common. In the other direction, as the manganese
decreases, wolframite grades into ferberite, a rather different-looking mineral, which is found in brillian t clusters of small black crystals in Boulder County, Colo.
ARSENOPYRITE
KEYS: Metallic luster. Can scratch glass. Gray or black
color.
Streak: Black. Fracture: Uneven.
Specific Gravity: 5.9-6.2 (medium weight).
The most common arsenic mineral, arsenopyrite is an
arsenide-sulfide of iron (FeAsS). Before it was given its
present name in 1847 it was known by the peculiar name
of mispickel. Large crystals from Tavistock. England, are
noteworthy. The fine crystals at Franconia, N.H., are
also familiar to collectors. Rounded masses as much as 1
foot in diameter, having alternating shells of niccolite,
come from Natsume, Japan. At Deloro, Ont., arsenopyrite
contains enough gold to make it valuable for that metal
alone. Cobalt and silver are other metals that have been
recovered from arsenopyrite. Thousands of other mining
districts throughout the world yield specimens of arseno·
pyrite, which is one of the first minerals to form. In the
Tres Hermanes Mountains of New Mexico the crystals
have grown as twins, in the shape of crosses and stars.
Some of the crystal groups look like marcasite, from
which they can usually be distinguished by their silverygray color, unless they have tarnished somewhat yellow;
moreover, unlike marcasite, arsenopyrite never has ~
fibrous structure. It seems to have a harsh appearance.
8a
CHROMITE
KEYS: Metallic luster. Can scratch glass. Gray or black
color.
Streak: Dark-brown. Fracture: Uneven.
Specific Gravity: 4.6 (medium weight).
Iron black --~
In composition an iron chromite (FeCr 2 0 4 ) and in use
the only ore of chromium, this mineral has an easy name
to remember. Good crystals, which resemble those of
magnetite and franklinite, are small and scarce; they are
found, for instance, on the northern islands of Unst and
Fetlar in the Shetland Islands. Chromite may be slightly
magnetic but not enough to be mistaken for magnetite.
A pitchy luster and a brownish cast to the color, while
not necessarily present, are frequent enough to aid in
recognizing chromite when you see it. Though hardly to
be called a North American mineral, some of it has been
mined along both coasts from Alaska to California and
from Newfoundland to North Carolina. Meteorites
sometimes contain chromite, which is typically a hightemperature mineraL Bricks made from it are so heatresistant that they are used to line the walls of furnaces
in which metals are melted. Chromite is extremely tough
and hard to break, and it is more than durable enough to
become a constituent of placer gravels, especially those
that contain platinum, for the two minerals are formed
and preserved under similar conditions.
ILMENITE
KEYS: Metallic luster. Can scratch glass. Gray or black
color.
Streak: Bleck or brownish-red. Fracture. Even.
Specific Gravity: 4.7 (medium weight).
Granular
An iron titanate (FeTiO s), ilmenite was named after the
lImen Mountains, Russia, the original locality. It has
come a long way from being an undesirable impurity in
iron ore to its present status as one of the most promising
industrial minerals of our time. The enormous new
deposit at Allard Lake. Que., contains enough ilmenite
to supply the whole world. Interestingly. this very black
mineral makes the whitest of all paints and the white
smoke used for skywriting and smoke screens. Crystals of
ilmenite that are bright or well-shaped are rare; they may
look like dark hematite. Some oc~ur in Orange County,
N.Y., and crystals weighing over 16 pounds have been
found at Kragero, Norway. Crystals are also found at
Chester, Mass., and at St. Christophe. France. The black
sand of certain beaches contains large amounts of ilmenite; for example, at Pablo Beach. Florida, and on the
extraordinary beaches, one of which extends for 15 miles,
in Travancore, India. Massive ilmenite resembles magnetite but is nonmagnetic, although the two minerals are
often intergrown so that they must be separated magnetically before they can be used.
84
MALACHITE
KEYS: Nonmetallic luster. Leaves colored mark on streak
plate. Green or blue mineral color.
Hardness: 3'12-4. Fracture: C onchoidal.
Specific G ravity: 3.9-4. 1 (heavy).
Often
velvety
4'-1
Pale green
streak
Malachite is a basic copper carbonate [Cu 2 (OR) 2 (COs) ].
and fizzes when touched by acid. Over one-half million
pounds of the choice green mineral were taken from one
pure mass of malachite in the Demidoff copper mine at
Nizhni Tagil, in the Ural Mountains of Siberia. It was
extensively used in Czarist Russia for table tops and
other ornamental purposes. Another form of Russian
malachite occurs in fibrous nodules having a silky luster;
these were carved into exquisitely fashioned objects of
art. The best crystals are found at Betzdorf, Germany,
and the finest of those that have changed over from blue
azurite come from Tsumeb, South-West Africa. The largest deposit of malachite is in the Katanga district of the
Belgian Congo and Northern Rhodesia, where valuable
copper ore is mined. as well as superb pieces of banded
malachite well suited to being cut and polished. New
Mexico and Arizona are the main producing states in the
United States, although malachite can be found in the
upper levels of almost every copper mine in the West.
The green patina that appears on copper and bronze is in
reality a thin tarnish of malachite.
AZURITE
KEYS: Nonmetallic luster. Leaves colored mark on streak
plate. Green or blue mineral color.
Hardness: 3'12-4. Fracture: Conchoidel.
Specific Gravity: 3.7-3.8 (heovy).
Blue
Often
velvety ___"Wl~~
~-~
_Conchoidal
fracture
Liqht blue streak
Azurite, like malachite, is a basic copper carbonate
[Cus (OH)2 (COS)2]. It fizzes similarly in contact with
acid. Its rich blue color, from which it receives its name,
is as distinctive as the equally deep green of malachite;
the two minerals often occur together in a wonderfully
appealing combination of contrasting colors. Inasmuch
as azurite readily changes to malachite, the result may
be a crystal which retains the original shape of the azurite
but which now has the green color and all the other
properties of malachite. The perfect and lustrous crystals
of azurite from Chessy, France, are the finest ever known,
though Tsumeb, South-West Africa, has more recently
become the chief locality for crystals of remarkable size
and beauty. Arizona is the leading American state, but
unfortunately most of the Bisbee copper mines that produced splendid crystals, singly and in groups, have now
been deepened below the zone in which azurite exists.
During the fifteenth and sixteenth centuries azurite was
the most popular blue pigment among European painters, and it had been employed for the ancient wall paintings of the Orient.
86
CHRYSOCOLLA
KEYS: Nonmetallic luster. leaves colored mark on streak
plate. Green or blue mineral color.
Hardness: 2-4. Fracture: Conchoidal.
Specific Gravity: 2.0-2.2 (light).
Conchoida~
fracture
~--"'>.,
'~c:===::-::::I
Pole green or
blue streak
A hydrous copper silicate of indefinite composition
[ (CuSiOs.nH 2 0)1. chrysocolla is a minor ore of copper. It
is used as an ornamental stone. substituting for turquoise,
which likewise varies between greenish and bluish hues.
Although softer and not so well known or so valuable as
turquoise. it is in some ways more durable because its
delicate color is permanent. and the shining enamellike surface which it often shows is quite attractive.
Chrysocolla occurs in the upper part of copper mines,
associated with other colorful minerals containing that
metal. Excellent specimens come from various localities
in Arizona. New Mexico, Chile. England, and the Belgian
Congo. The only crystals that have ever been found were
from Mackay, Idaho. and are of microscopic size. The
word chrysocoUa, meaning "gold glue," was given to a
similar-looking mineral used in olden times to solder
gold, and it was later transferred in error to this mineral.
When impure, as it often is because of its somewhat variable content of water, chrysocolla becomes brown or
black and has then been called pitchy copper ore.
RUTILE
KEYS: Nonmetallic luster. Leaves colored mark on streak
plate. Red or orange mineral color.
Hardness: 6-6 112. Fracture: Uneven.
Specific Gravity: 4.2-4.3 (heavy).
Striations
~~i--Cllon9 length
~Palebrown
~
streak
Rutile is the most familiar of the three minerals that are
composed of titanium oxide and share the same formula
(Ti0 2 ) , the others being anatase and brookite. The
name rutile is derived from the Latin word for red, in
reference to the color. The splendid specimens from
Graves Mountain, Ga., are the finest in the United
States. They consist of twinned crystals which repeat
themselves until they make a complete ring. Magnet
Cove, Ark., Stony Point, N.C., and Laws, Calif., are other
outstanding American localities. Well-crystallized rutile
comes from veins and rock crevices in a number of places
in the Alps. Dark rutile crystals look a good deal like
those of cassiterite, but they are not so heavy. Rolled
pebbles are found in the diamond-bearing gravels in
Brazil, and commercial deposits, where rutile is obtained
as an are of titanium, are found in the sand on Florida
beaches. Needlelike red crystals of rutile are common as
inclusions in quartz, which is then known as rutilated
quartz; sagenite, a favorite of mineral collectors, is a related quartz material named from the network of crystals
buried within it.
88
CINNABAR
KEYS: Nonmetallic luster. leaves colored mark on streak
plate. Red or orange mineral color.
Hardness: 2-2'12. Fracture: Uneven.
Specific Gravity: 8.0-8.2 (very heavy).
Adamantine
luster--~/
~ed--:l.J
Scarlet
streak
......
The chief ore of mercury (quicksilver) is cinnabar, mercury sulfide (HgS). Low-grade deposits are often deceiving because a thin crust of the bright red mineral gives
a false appearance of richness. Impure cinnabar may be
dark-red, almost black_ Cinnabar forms near hot springs
and is found near the earth's surface in areas of recent
volcanic rocks. The most extensive occurrence in the
world is at Almaden, Spain. Another large one is at Idria,
Italy. Named after these ancient localities are the important California mines at New Almaden and New Idria.
The mercury produced at New Almaden is credited with
having made possible the tremendous gold production
that took place during the California gold rush of the
years immediately after 1848. At Terlingua, Tex., some
of the cinnabar has altered to a group of rare mercury
minerals. Pebbles of cinnabar, rolled round by stream action, are panned in gold placers in Dutch Guiana. Large
crystals are obtained in China, and excellent ones also
come from near Belgrade, Yugoslavia, and in Pike
County, Ark. These show a pronounced cleavage not obvious in the usual finely granular material.
89
REALGAR
KEYS: Nonmetallic luster. l.eaves colored mark on streak
plate. Rea or orange mineral color.
Hardness: 1112-2. Cleavage: 1 direction.
Specific Gravity: 3.5 (medium weight).
Often
ye II ow spots
Cleava
Orange streak
~
Realgar is arsenic sulfide (AsS) in its simplest form. It
is not too stable a mineral, and exposure to light, which
should be avoided, causes it to change rapidly from red
to orange, eventually becoming yellow orpiment. The
two minerals are almost always found together in nature.
Besides ore veins, hot springs and volcanoes are the chief
sources of realgar. Good crystals, otherwise rare, come
from Mercur, Utah; the Binnental, Switzerland; and the
island of Corsica in the Mediterranean. Fine specimens of
this attractive mineral are also found at Manhattan,
in Nevada. In Rumania it occurs with ores of silver and
lead, occasionally in lovely translucent crystals. Nests of
realgar are sometimes clustered in beds of clay. Thin coatings of realgar are deposited around Yellowstone geysers
(in the Norris geyser basin) and Italian steam vents. The
curious word realgar is derived from an Arabic term,
yah; al-ghaT, meaning "powder of the mine." The natural mineral was formerly used as a pigment and in fireworks, but is no longer. Realgar is softer and lighter in
weight than cinnabar, which it may superficially appear
to be.
go
ORPIMENT
KEYS: Nonmetallic luster. Leaves colored mark on streak
plate. Yellow mineral color.
Hardness: I V2-2. Cleavage: I direction.
Specific Gravity: 3.5 (medium weight).
Yellow----_~---~
Yellow sheak
~
Orpiment, like realgar, is arsenic sulfide but has a different formula (As 2 SS )' Its tabular crystals are very uncommon and almost always small. Those from Mercur, Utah,
however, are exceptionally large and fine; excellent ones
also have come from Balin, Russia. Substantial amounts
of orpiment occur at Acobambilla in Peru, and Julamerk
in Turkey. The hot water at Steamboat Springs, Nev.;
Yellowstone National Park, Wyo.; and Shimotsuke,
Japan, deposits orpiment from springs and geysers onto
the surrounding rocks, as do smoke vents in Italy. Lemonyellow orpiment is often seen as an alteration product on
pieces of realgar, and it is frequently scattered through
certain layered rocks. At some places it has resulted from
fires in a mine. Its name comes from the Latin name
which means "golden paint," in reference to its color and
because it was believed to contain gold-a compliment,
indeed, to its vivid color. A little natural orpiment is used
as a dye and in tanning skins, but the main use of the
mineral is as specimens for collections. The handsome
combinations of orpiment and realgar add life to any
display.
CARNOTITE
KEYS: Nonmetallic luster. Leaves colored mark on streak
plate. Yellow mineral color.
Hardness: 1-2. Fracture: Earthy.
Specific Gravity: 5.0 (heavy).
Yel
~
Yellow streak
Carnotite is a complex mineral containing uranium, vanadium, and radium. Its chemical formula is close to K 2 (U02h (VO.)2.3H20. All three of the metals mentioned
have in the past been extracted from it on a commercial
scale. First discovered in Colorado, it was taken to France
for analysis and was named there after a French mining
engineer and chemist, Marie-Adolphe Carnot (18391920). It and pitchblende are significant ores of uranium
in the United States, and this is now its sole use. In the
remote plateau country of the Four Corners region-the
only place in the nation where four states come together
(Colorado, Utah, Arizona, New Mexico) -carnotite occurs uniquely in sandstone, associated with dinosaur
bones, petrified wood, and vegetable matter. Two petrified logs from Calamity Gulch, Colo., and the rock between them yielded $350,000 in the three valuable metals.
A little carnotite is known in other states, including
Pennsylvania, and it forms a thin film on rock at Radium
Hill, South Australia. Tiny micalike crystals can be expected at times, but a powdery or loosely granular form
is the usual aspect.
9t
SPHALERITE
KEYS: Nonmet allic luster. l eaves colored mark on streak
plate. Brown mineral color.
Hardness: 3.5-4.0. C leava ge: 6 directions.
Specific G ravity: 3.9-4.2 (heavy).
Resinous
Zinc sulfide (ZnS), sphalerite, is the principal ore of zinc.
Because it assumes various appearances, sphalerite is
sometimes difficult to recognize and may be mistaken for
minerals that bring a higher price; from this risk of disappointment it has obtained its name, which means
treacherous. The name preferred in England is blende,
which has a similar significance. Miners use additional
terms, including zinc blende, blackjack, ruby zinc, and
still others. Pure sphalerite is colorless, but iron is nearly
always present, giving it a color that ranges from yellow
to brown to black and may even be green. The world's
greatest deposit of sphalerite is in the Tri-state area where
Missouri, Kansas, and Oklahoma corne together; the
specimens from here are numerous and attractive. Some
are as radiant as rubies. Butte, Mont., has also furnished
remarkably good specimens. Fine groups occur at Alston
Moor, England, and other localities in that country. Large
masses of gorgeous golden color occur at Picos de Europa,
Spain, and Cananea, Mexico. Unusually pure sphalerite
has been analyzed from the Prince Frederick mine in
Arkansas.
9!J
CASSITERITE
KEYS: Nonmetallic luster. Leaves colored mark on streak
plate. Brown mineral color.
Hardness: 6-7. Fracture: Uneven.
Specific Gravity: 7.0 (very heavy).
Pole brown
streak
!
~
The oxide of tin, cassiterite (5n0 2 ) , is the most important ore of tin. The vast deposits which stretch for 1,000
miles from Burma and China to the Netherlands East
Indies yield cassiterite in large amounts, and it is also
mined in Bolivia, Nigeria, the Belgian Congo, Australia,
and a few other countries. Great Britain, known to an·
cient geographers as the Cassiterides or Tin Islands, was
long a significant producer. Perhaps the most unusual
occurrence of this mineral are the deer horns found to
have changed to cassiterite in the streams of Cornwall,
England. Odd shapes exist, including sparable tin (like
a cobbler's nail) , wood tin (resembling a piece of dried
wood), and toad's-eye tin. Because they are so often
found in placer deposits, pebbles of cassiterite are popularly known as stream tin. Perfect crystals come from the
tin mines of both the German and Czech sides of the
mountain range called the Erzgebirge. Cassiterite is
uncommon in the United States, marking a serious mineral deficiency, though it is found in a dozen scattered
states, especially in the Black Hills of South Dakota.
94
MICA
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Shows good cleavage. Can be scratched
by fingernail.
Color: White , green = muscovite. Black, dark-brown ==biotite. Yellowish-brown = phlogopite. Pink, purple =
lepidolite.
Specific Gravit y: 2.7-3.3 (medium weight, but may float
on water because flaky).
Mica is a group of minerals which are potassium-aluminum silicates and give off water when heated. They
all have the remarkable property of peeling into thin
sheets which not only can be bent but are also elastic,
springing back to their original position when released,
unless they have been carelessly broken. Thick crystals are
called books, as they resemble a volume having many
leaves or pages. The several kinds of mica have different
colors because of variations in the chemical formula. Muscovite, also called white mica, has the simplest composition. Crystals of muscovite 33 feet long have been
found in Ontario. The replacement of aluminum by
magnesium produces phlogopite or brown mica; when
both magnesium and iron are present, the mineral is
biotite or black mica. Lepidolite or lithia mica has a
lovely pink or purple color. White or light·brown mica,
familiar as isinglass, is a strategic mineral for insulation.
95
CHLORITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Can be scratched
by fingernail.
Color: Green. Specific Gravity: 2.6-3.0 (medium weight).
Chlorite is a group of hydrous silicate minerals which
resemble green mica except that they are not elastic.
Their curved leaves can be bent but do not return to the
original position, as do the flat flakes of true mica. They
seem to have a slightly soapy feel. The individual members of this group go under such names as clinochlore,
penninite, and prochlorite; it is usually not necessary to
try to distinguish among them. Their chemical formulas,
as is the case with so many of the silicate minerals, are
very complex. Chlorite makes up the mass of certain
widespread green rocks and frequently dusts the surface
of other minerals or occurs inside them. At Zermatt in
the Swiss Alps are large green crystals of chlorite, which
are especialJy noted among mineral collectors. Substantial
plates are found at West Chester, Pa. Garnet is one of a
number of minerals which occur in chlorite and alter to
it; many of the big crystals from near Salida, Colo., have
"rotted" to chlorite almost all the way through. The word
chlorite means green, and this mineral is responsible for
the color of many rocks, as a result of weathering and
e action of hot water.
96
GYPSUM
KEYS : Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Can be scratched
by fingernail.
Color: W hite, gray. Specific Gravity: 2.2-2.4 (mediu m
weight).
Calcium sulfate (CaSO •. 2H 2 0) in composition, gypsum
is the most common mineral of the sulfate class. Thick
beds are known over large areas of the globe, supplying
the basic raw material for making plaster of paris. This
product gets its name from the gypsum deposits in the
Paris basin in France. The crystals from Naica, Mexico,
are of colossal size, and those from Wayne County, Utah,
and Ellsworth, Ohio, are likewise exceptionally large.
Among other clear and nicely formed crystals of gypsum
are those from the salt mines at Bex, Switzerland, and
Girgenti, Sicily. Transparent gypsum like this is called
selenite. Compact material solid enough to be carved into
ornaments and useful articles is familiar as alabaster, of
which northern Italy and Colorado are the leading
sources. Gypsum in veins composed of silky fibers is called
satin spar, which has been cut into beads and sold for
many years at Niagara Falls. The White Sand~ in New
Mexico, near the rocket proving ground, consist of windblown dun es of gypsum sand. Earthy gypsum is called
gypsite.
97
TALC
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Show$ good cleavage. Can be scratched
by fingernail.
Color: Green, white.
Specific Gravity: 2.7-2.8 (medium weight).
This hydrous magnesium silicate [Mgs (OH) 2 Si.0 1o] is
closely related to serpentine. Although the origin of the
word talc is lost in obscurity, everyone knows the most
familiar product made from this mineral-talcum powder. Owing to its being the softest of all minerals, talc
feels soapy. A somewhat more solid form, called soapstone
or steatite, is used for chemical sinks and electrical switchboards. Small pieces, called French chalk, are used by
tailors to mark cloth. Most of the low-priced Objects of
Chinese manufacture thal are supposed to look like jade
are really soapstone, and many of the rest are pyrophyllite, a related mineral grouped with talc when statistics
of production are given. Beautiful sea-green specimens of
talc come from various places in the Alps; talc from the
French Pyrenees is the finest for the cosmetic industry.
New York, California, and North Carolina are the leading American sources for talc, which is mined, at intervals, the length of the Appalachian Mountains. Talc combined with other minerals that have grown in it makes
interesting specimens.
98
CALCITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good deavage. Cannot be
scratched by fingernail, but can be scratched by copper coin.
Color: White, colorless.
Specific Gravity: 2.7 (medium weight).
May be tinted
or darkened
/
Rh ombic
.cleavoqe
Calcite is calcium carbonate (CaCOs ), often very pure.
Like all carbonates it fizzes in acid. Describing typical
calci te would be difficult. for over 300 forms and more
than 1.000 combinations have been recorded. Hence calcite may have almost any outward appearance. but it
always breaks into fragments of one shape, the rhomb.
Fortunately. some kinds of calcite crystals turn up frequently enougb to become familiar. Absolutely clear
pieces of calcite, first found in Iceland, are called Iceland
spar; tbey have the interesting property of making a line
seem double when viewed through them. One crystal of
this variety from near Helgustadir. Iceland. measured .20
feet in length. Several crystals weighing more than 25 tons
each have come from near Taos. in New Mexico. The
golden calcite from Joplin. Mo.• is little short of spectacular. as are the resplendent crystals from several localities
in northern England. "Dogtooth spar" and "nailhead
spar" owe their names to peculiar forms of calcite crystals.
99
DOLOMITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by fingernail. but can be scratched by copper coin.
Color: Pink, white.
Specific Gravity: 2.8-2.9 (medium weight).
Rhombic
cleavage
--=+
Curved face.s •
Pearly luster
~
/'
~
.
The calcium-magnesium carbonate [CaMg (COs) 2] is
called dolomite after Deodat de Dolomieu (17 50-1801),
a French engineer and mineralogist. Being a carbonate, it
fizzes in acid, though less vigorously than calcite and
though the acid may have to be warmed. Beds of dolomite
of considerable thickness have been deposited in many
parts of the world, where they may be used as building
stone. Ordinary limestone turns to dolomite upon the
addition of magnesium from anyone of a number of
underground sources. Large transparent dolomite crystals
of choice quality occur near Djel£a, Algeria. Dolomite
grows on the inside of the intriguing hollow rocks which
are so abundant near Keokuk, Iowa; called geodes, they
resemble rough, rocky balls until they are broken open,
when they reveal a glitter of many tiny crystals. Large
crystals of dolomite come from Roxbury, Vt. The mineral
is found in cavities in rock at Niagara Falls, Lockport, and
Rochester, N .Y. A good deal of translucent dolomite surrounds the lead and zinc minerals at Joplin, Mo.
100
BARiTE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by fingernail, but can be scratched by copper coin.
Color: White, colorless, blue.
Specific Gravity: 4.5 (heavy).
Barite is barium sulfate (BaSO.), the most common mineral containing barium and one of the most common sulfate minerals. Its name appropriately means "heavy."
Crystals of an enchanting blue color can be collected in
abundance at two localities in Colorado, near Hartsel and
Sterling. Reddish-brown flowerlike groups, such as those
from Norman, Okla., are called barite roses. A single
transparent crystal weighing 100 pounds was taken from
a lead mine in Westmorland County, England. Large
quantities of white barite, carrying brown stains in cracks
and corners, are typical of the commercial deposits in
Missouri. Stalactites of brown barite are found at Newhaven, England. In the Bad Lands of South Dakota crystals of barite have grown in distorted shapes inside the
h ollow bones of fossil animals buried there for millions
of years. Barite localities are numerous in California. For
its home this mineral favors limestone and metal-bearing
veins. Barite is used in house paint and in many other
ways.
101
CELESTITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by fingernail, but can be scratched by copper coin.
Color: Blue, colorless, white.
Specific Gravity: 3.9-4.0 (heovy).
_,,- Diomond-shoped
cleovoqe
~~
,..
Celestite is strontium sulfate (SrSO.) and was given its
name because the first specimens, found near Bellwood,
Pa., were a celestial blue. This color, though frequent
enough in most places, is not an essential characteristic.
however. In a cave at historic Put in Bay in Lake Erie
excellent crystals over 18 inches long have been discovered. Daintily fashioned groups of crystals are collected at
Clay Center, Ohio. The celestite in the sulfur beds at
Girgenti, Sicily, is doubtless the best known. Complex
blue crystals occur in cavities in copper mines at Herrengrund, Czechoslovakia. Large crystals are found at Lampasas, Tex. Some fossils are seen to have changed to
celestite as a result of petrifaction under special geologic
conditions. This mineral and barite often look exactly
alike, especially when they have a tinge of blue. Then
perhaps the best way to tell them apart is to hold a piece
in a very hot flame, which will become green for barite or
red for celestite when the mineral begins to decompose.
102
HALITE
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratc~ed by fingernail, but can be scratched by copper cOin.
Color: C olorless. wh ite. Specific Gravity: 2.2 (light).
Often
hopper-shaped
Vitreous
luster ;..._-......
Movable bubble
As a mineral, sodium chloride (NaCl) is called halite,
but it is still the familiar table salt. Vast beds of such
rock salt, and huge bodies called salt domes abound.
Well-crystallized specimens have come from Stassfurt,
Germany, and Bochnia, Poland, as well as several places
on the island of Sicily. Peculiar distorted crystals are
found in Humboldt County, Nev. Attractive masses of
superior transparency are obtained in the Verde Valley,
Ariz. A curious feature of many halite crystals is the
hopper-shaped opening that penetrates the faces . Interesting, also, are the occasional hollow Spots inside, containing a drop of water which moves back and forth like
the bubble in a carpenter's spirit level. The salty taste of
halite is a distinctive property of it, and of course it dissolves easily in water. Some halite is blue, possibly owing
to the displacement of a toms in the structure. Contrary to
general belief, most hali te is used in the chemical industry
rather than as food, but its use to sustain life cannot be
dispensed wi th.
CRYOLITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by fingernail, but can be scratched by copper coin.
Color: Colorless. white.
Specific Gravity: 3.0 (medium weight).
t (L.. _
Almost invisible
in woter
lMelts in match flame
Natural sodium-aluminum fluoride (Na s AIF6) is cryolite.
The early Norsemen who visited Greenland were astonished to find the Eskimos using as anchors a heavy stone
which disappeared when lowered into the sea. Cryolite
has the strange property of being almost invisible in
water, because it has nearly the same light-bending power
as water, and rays of light pass through it in a straight
line. Its name means "frost stone," in allusion to its icy
appearance. It can be melted in a candle flame. The occurrence of cryolite in Greenland is the more remarkable
because the mineral is extremely rare except there; but
at Ivigtut in Arksuk Fjord on the west coast it is found
in an enormous deposit. Associated with the cryolite are
a number of rare fluorine-bearing minerals derived from
it, and some common ore minerals. The cryoli te is taken
to Denmark for processing, and it is used as an insecticide
and in the aluminum industry. Very limited amounts
have also been found near Miask, Siberia; Sallent, Spain;
and Colorado Springs, Colo.
104
ANHYDRITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by fingernail, but can be scratched by copper coin.
Color: White. Specific G ravity: 2.9-3.0 (medium weight).
Pea rly luster
on cleavage ---.p,
Its composition being calcium sulfate (CaSO.), anhydrite
in 1803 received its name because it does not contain
water, as does gypsum, which otherwise has the same
chemical formula. At Lockport, N.Y., fine blue anhydrite
is found in cavities in limestone, and good crystals occur
at Stassfurt, Germany. At limes anhydrite appears in veins
with metallic minerals and in the gas cavities of volcanic
rocks. It occurs mostly, however, in large masses, together
with rock salt and gypsum, which are likewise deposited
by the evaporation of sea water in isolated arms of the
ocean. Either anhydrite or gypsum will precipitate from
the same solution, according to the temperature of the
water, the salt content, and other factors; afterward anhydrite may change into gypsum by adding water, or gypsum may change into anhydrite by losing it. Thick beds
of anhydrite are situated near Carlsbad National Park,
N.M., and in the adjacent part of Texas. Nova Scotia also
contains extensive beds. On Calumet Island, Que., anhydrite h as replaced marble of very great geologic age.
10 5
KERNITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by fingernail, but can be scratched by copper coin.
Color: Colorless, white. Specific Gravity: 1.9 (light).
Surface
often
cholky
Kernite, a hydrous sodium borate (Na2B.07.4H20), is
now the chief commercial source of borax. Found in 1926
and named after Kern County, Calif., kernite was not
known anywhere else in the world. It occurs near Kramer
in the Mojave Desert, in a deposit containing millions of
tons. This is the unusual circumstance of a new mineral's
being discovered in huge quantities at the start. Kernite
is associated with the mineral borax, from which it is
believed to have altered by contact with heated rocks that
forced themselves up into a buried lake from beneath.
The largest single crystal measured 8 feet long and 3 feet
wide, and many others are several feet thick. When in
contact with other borate minerals, the surface of kernite
turns chalky white, but isolated specimens do not behave this way. This coating is tincalconite, named from
the ancient Oriental word for borax powder. When
placed in water, kernite becomes white and opaque, and
then dissolves-slowly in cold water, more rapidly in hot
water.
106
ANGLESITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. C annot be
scratched by fingernail . but can be scratched by copper coin.
Color: C olorless, white, grey.
Specific Gravity: 6.2-6.4 (very heevy).
Anglesite, which is lead sulfate (PbSO.), results from
the oxidation of galena and is often recognized as a gray
band enclosing a core of still-shiny galena, which has
been preserved from change. Another lead mineral which
forms at the same time is cerussite, which may in tum
alter from the anglesite. Interesting to find are cavities
in galena lined with crystals of anglesite and sulfur.
Though much less important than galena, anglesite is
also used as an ore of lead. Large amounts are mined in
Mexico and Australia. Crystals are embedded in sulfur at
Los Lamentos, Mexico. Superlative crystals have come
from Sidi-Amor-ben-Salem, Tunis. The crystals from
Monte Poni, Sardinia, are also of exciting quality. The
original locality after which the mineral was named is the
island of Anglesey in the Irish Sea; but the specimens
there were small, although good ones have been obtained
from Pary's mine. In the United States the largest are
those labeled Wheatley mine, Phoenixville, Pa.
10']
SODA LITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Blue. Specific Gravity: 2.2-2.4 (medium weight).
Vitreous
Sodalite is a sodium-aluminum silicate (Na4AIsSis012CI) . A test for the chlorine it contains is often needed for
an accurate identification of this mineral, particularly if
it happens not to be blue. The name, adopted in 1811,
refers to the sodium that is present. The rich blue color
of sodalite makes it a satisfying ornamental stone. Masses
of this select quality occur in Canada on Ice River near
Kicking Horse Pass, B.C.; Dungannon, Ont.; and several
places in Quebec. Litchfield, Me., and Salem, Mass., are
American sources. Unusual transparent white crystals
are found in the lavas of Mount Vesuvius. Sodalite is a
member of the feldspathoid group of minerals, so called
because they take the place of feldspar in certain rocks
that are deficient in silica. Other feldspathoid minerals
that also have a deep-blue color, but are less abundant,
are lazurite, noselite, and hauynite-this last one constituting most of the gemmy rock known as lapis lazuli,
which is marked by white streaks of calcite and golden
flecks of pyrite and was known to antiquity as sapphire.
108
PYROXENE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: BlocK, green, white.
Specific Gravity: 3.2-3.4 (medium weight).
Often one
par tit19
Pyroxene is one of the most important groups of minerals,
the essential constituent of many rocks formed at high
temperatures. The members of the group are all silicates
but vary in composition considerably among themselves.
Jadeite, the preferred of the tw kinds of true jade, is a
pyroxene found mainly in Upper Burma. Spodumene is
remarkable because of its enormous crystals, some individuals from near Keystone, S.D., measuring 40 feet or more
in length and weighing up to go tons. A delicate variety
of spodumene, cut into lovely pinkish-violet gems, is
kunzite, found mostly at Pala, Calif., and in Madagascar.
Diopside, another pyroxene, occurs largely in marble, as
do the clear crystals at Ala, Italy. Enstatite is common in
meteorites, and grades into hypersthene as the percentage
of iron increases. Of all the pyroxenes, however, the one
by far the likeliest to turn up is augite; hence this is the
one illustrated above. Crystals of it are common in the
volcanic cinders of the Hawaiian Islands and elsewhere.
109
AMPHIBOLE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Black, green, white.
Specific Gravity: 2.9-3.4 (medium weight).
.._-
/~~
Two cleovoCje5
Amphibole is a group of minerals related to pyroxene
and of great importance as constituents of rocks. Unlike
the pyroxenes they yield some water when sufficiently
heated, and their crystals tend toward being six·sided
rather than squarish. In composition they are complex
silicates, hornblende, the most common of them, having
a chemical formula nearly as long as a line of type in this
book. Hornblende is taken as the typical amphibole and
is illustrated here, but the other minerals of the group
have the same oblique cleavage and are thereby distinguished from pyroxene. Arfvedsonite is a sodium-rich amphibole found in large crystals near Julianehaab, Greenland. Glaucophane, occurring in the Coast Ranges of
California, has also a high content of sodium but a distinctive blue color. Anthophyllite, named from the Latin
word for clove because of its clove-brown color, is one of
the simpler amphiboles. White tremolite grades into
green actinolite; when either of them is compact and
tough, it is called nephrite, which is a true jade.
110
KYANITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Blue, white.
Specific Gravity: 3.5-3.7 (medium weight).
An aluminum silicate (AI 2Si0 3) , kyanite has a chemical
composition identical with those of andalusite and sillimanite. When heated to the neighborhood of 10OO°C., it
becomes a substance called mullite, which furnishes a
heat-defying and shock-resisting porcelain used for spark
plugs and chemical ware. The variable hardness of kyanite, whereby it can be scratched by a knife along the
"grain" but resists being scratched in the perpendicular
direction, is a unique characteristic. Its attractive bicolored, blade-shaped crystals are the principal reason, however, for its strong appeal to the mineral collector. The
name kyanite refers to its blue color, which is often in
spots or streaks. Beautiful combinations of kyanite and
staurolite occur at Monte Campione, Swiuerland. The
clear green crystals from Yancey County, N. C., are different but none the less lovely. The chief commercial
deposits are at Lapsa Bura, India, and in Kenya, the
British colony in East Africa.
III
FLUORITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Purple, light-green, yellow.
Specific Gravity: 3.2 (medium weight).
Octahedral
~cleovGlge
Calcium fluoride (CaF 2 ) is called fluorite. Although it is
the mineral that gave its name to fluorescence, this interesting effect is weakly shown. Superb crystal groups come
from noted English localities, including Castleton in
Derbyshire and Cleator Moor in Cumberland. From
Derbyshire, also, came the prized blue john, a banded
blue variety which was carved into vases. Large sea-green
cubes are found at Muscalonge Lake, N.Y. Pretty brown
crystals come from Clay Center, Ohio. The gorgeous
purple specimens from near Rosiclare, Ill., are indeed
outstanding; some of the large, clear crystals look almost
like stained-glass church windows when the sun gleams
through. Clear, colorless crystals come from Modoc, Onto
Scarcely any lovely hue-plum, rose, any you prefer-is
missing from the colors that fluorite may possess. This
mineral received its name from the Latin word meaning
"to flow" because it melts at a low temperature; today its
main use is as a flux to help melt iron ore when making
steel.
112
RHODONITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Rose-red. pink. brown.
Specific Gravity: 3.4-3 .7 (medium weight).
Two
Oft en brown
or black spots
Cleavages
Given its name from the Greek word for a rose, on account of its rose-pink color, rhodonite is manganese silicate (MnSiO s ). Large quantities of it have been mined
near Sverdlovsk (formerly Ekaterinburg) in the Ural
Mountains and were extensively used in old Russia as an
ornamental stone. Among the best-known sources of rhodonite crystals are Franklin, N.].; Langban, Sweden; and
Broken Hill, New South Wales. The New Jersey variety
contains zinc and is also called fowlerite. The black coating on the surface of typical specimens is the result of an
external alteration to manganese oxide, which may extend along cracks within the mineral. Rhodonite is a
minor ore of manganese, generally when it is associated
with other minerals of greater economic importance.
Rhodonite looks like pink feldspar, which is lighter in
weight; and somewhat like rhodochrosite, which is a carbonate and has a different cleavage. It also resembles the
pyroxene minerals in crystallization but is no longer regarded as belonging to the same group.
RHODOCHROSITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Rose-red, pink.
Specific Gravity: 3.3-3.6 (medium weight).
Manganese carbonate (MgCO s) is rhodochrosite, so
named because of its lovely rose-red color. Like other
carbonate minerals it fizzes when a drop of acid is applied,
and finally dissolves completely in warm acid. Rhodochrosite is an important ore of manganese at Butte, Mont.,
but hac only a little use elsewhere. Unrivaled crystals.
almost ruby-red in color and clarity, have come from old
silver mines in central Colorado. Rhodochrosite is a common mineral in the silver mines at Austin, Nev. It is also
found elsewhere in the United States, especially at
Branchville, Conn., and Franklin, N .J. In Europe the
leading localities include Kapnik, Rumania. and Frei·
berg. Germany, where it occurs in silver veins. Rhodochrosite is softer than rhodonite, which likewise has a
pink color. A black or brown surface covering indicates
chemical alteration. Apart from its color, it has the appearance of calcite, dolomite, siderite, and smithsonite.
which are all carbonate minerals belonging to the same
group and having the same rhombohedral cleavage.
114
SIDERITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Brown. Specific Gravity: 3.8-3.9Iheovy).
Iron carbonate (FeCOs) is known as siderite from the
Greek word meaning iron. Spathic iron and chalybite are
miners' names for this mineral. The tendency of siderite
crystals to seem curved is worth noting, for this property
is restricted to only a few minerals, including dolomite.
The specimens from many of the mines in Cornwall,
England, are exceptionally good; the six-sided plates from
Wheal Maudlin are evenly developed and highly attractive. Good crystals also occur at Brosso, Italy, and Allevard, France. A deposit of economic value is situated at
Styria, Austria, and siderite is also mined in Great Britain, but the proportion of iron is too low for it to be used
very much in most countries, where better ores are available. Veins of lead and silver ores in Idaho contain much
siderite. Impure siderite, mixed with clay and known as
clay ironstone, is distributed through the coal beds that
extend from Pennsylvania to Ohio. Siderite alters readily
to other minerals, especially goethite, which retain the
original siderite shape.
STRONTIANITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: W hite. Specific Gravity: 3.7 (medium weig ht).
Vitreous
luster---+
Two cle.avages
As carbonate of strontium (SrCO s), strontianite gave its
name in 1790 to the chemical element of which it is one
source of supply. Like other carbonate minerals it fizzes
in acid. The original locality was Strontian, Scotland, the
town after which it received its name. The biggest deposits are situated in Germany at Drensteinfurt, Ascheberg, and Ahlen, and these have yielded fine crystals.
Well-proportioned crystals occur in the lead mines at
Pateley Bridge, England; strontianite is sometimes associated in this way with metals in veins, more especially
in Germany and also in Mexico. Nests and spheres of
crystals are found at Schoharie, N.Y. Good-sized deposits
occur in the Strontium Hills, north of Barstow, Calif.,
and the mineral comes from other places in that state.
In Texas strontianite occurs on Mount Bonnell (near
Austin) and in the cap rock that overlies the salt domes
along the Gull of Mexico. In Washington it is collected
near La Conner. Strontianite resembles aragonite in its
over-all nature, except that it is mu ch less likely to form
in distinct crystals.
116
SMITHSONITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scrat ched by copper coin, but can be scratched by
knife blade.
Color: W hite, brown , green. Specific Gravity: 4.4 (heavy).
Smithsonite, zinc carbonate (ZnCO s), is one of the most
variable of all minerals in appearance. Some of its varieties are so disguised that they do not look a bit like
smithsonite. Take, for example, the so-called dry-bone
ore, which, of course, resembles a dried bone; it is mined
in the zinc district of Wisconsin-Illinois-Iowa. Another
curious variety is turkey-fat ore, yellow in color because
it contains cadmium; stalactites of it, with concentric
banding, come from Sardinia. White spheres of smith·
sonite are found in the mines in the province of Santander,
Spain. Probably the most appealing smithsonite is a rich
green, solid enough for gem cutting, and coming from
Kelly, N.M. Several of the smithsonite colors are associ·
ated together at Laurium, Greece. Matchless crystals have
come from Rhodesia. Large deposits of smithsonite occur
in Germany. This mineral was named in 1832 in honor of
the Englishman James Smithson (1765-1829), who
founded the Smithsonian Institution in Washington; in
England it used to be called calamine.
STILBITE
KEYS: Nonmm-allic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: White. Specific Gravity: 2.1-2.2 (light).
An aluminum silicate with a complex formula, stilbite
is one of the numerous attractive members of the zeolite
family, all of which are characterized by the fact that they
give off water in copious amounts at a steady rate when
they are heated. Its distinctive form is that of sheaflike
groups of crystals, which spread out at both ends. Single,
untwinned crystals are not known, but twins of stilbite
often resemble a tabular crystal-a common type of oc·
currence. The name stilbite comes from the Greek word
for luster, in reference to the pearly luster on the faces
that correspond to the cleavage surfaces. Stilbite occurs
with other zeolites in cavities in lava rocks which have
cooled upon or near the surface of the earth. Fine·quality
specimens of stilbite are numerous in northern New Jersey, in Nova Scotia, and at Guanajuato, Mexico. Large
salmon-colored tabular crystals are found at Poona, India.
Peerless white crystals of the same shape line cavities in
the basalt of the Faeroe Islands, in the North Atlantic
Ocean. The desirable crystals from Kilpatrick, Scotland,
are red.
118
HEMIMORPHITE
KEYS: Nonmetallic luster. Leaves
on streak plate. Shows good
scratched by copper coin. but
knife blade.
Color: Colorless, white.
Specific Gravity: 3.4-3.5 (medium
white mark or scratch
cleavage. C Cl nnot be
can be scratched by
weight).
Vitreous luster
Two cleavages
Hemimorphite, a hydrous silicate of zinc [Zn,Si 2 0 1 (OH) 2.H 2 0]. has been the innocent victim of a confusion in
names which has also involved other zinc minerals. Most
mining people still call it calamine, but the scientific
name of hemimorphite was suggested to apply solely to
this one mineral. It refers to the fact that the crystals are
developed in halves. the opposite ends being different
from each other. This sometimes produces a peculiar effect
of unbalance and lack of symmetry as the shea£like groups
are examined closely. and the pointed ends of the individual crystals are seen to be attached to the rock. The
glossy crystals from the old zinc mines at Aachen, Germany. are well known. Truly splendid crystals have come
from Djebel Guergour, Algeria. Elkhorn, Mont., and
Granby, Mo .• are American localities for fine specimens.
Hemimorphite serves as a significant ore of zinc. It is a
secondary mineral. originating from the action of silicabearing water upon other zinc minerals. Its closest companion is smithsonite.
119
NATROLITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Colorless, white. Specific Gravity: 2.2-2.3 (light).
This hydrous silicate of sodium and aluminum, with the
formula Na2A12Si30lO"2H20, is one of the zeolite minerals, with their property of becoming dehydrated at a
constant rate upon being heated. Its name alludes to its
sodium content. As one of the last minerals to solidify,
natrolite incrusts the surface of cracks and cavities in volcanic rocks, along with other zeolites and calcite. The
radiating groups of slender crystals are so typical that
natrolite has been called needle zeolite. It fuses more
easily than aragonite, for which it might be taken. Among
the largest of all known specimens are the dazzling white
crystals that used to come from Puy de Marman, France.
Large crystals come from British Columbia's Ice Valley,
and needles several inches long are known at Bishopton,
Scotland. Fine specimens occur in the trap quarries at
Weehawken and other towns in New Jersey, and in the
renowned zeolite-bearing rocks of Nova Scotia. Aussig,
Czechoslovakia, is another important locality for natrolite.
120
COLEMANITE
KEYS : Nonmetallic luster. leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Colorless, white.
Specific Gravity: 2.4 (medium weight).
Vitreou s luster
The handsomest of the borate minerals is colemanite, a
hydrous calcium borate (Ca2BoOu.5H20). The brilliant
clear crystals, up to several inches in length, often occur as
a lining inside porcelain like chunks of the same mineral.
The crystals look rather like calcite but have a brighter
luster. Rounded masses, however, are the usual mode of
occurrence. First noticed in Death Valley in 1883 and furnishing the main supply of i.he world's borax at the time
of the discovery of kernite in 1926, colemanite is found
as a buried lake deposit in a number of counties in
southern California and western Nevada. It is believed
to be an alteration of ulexite, a mineral sometimes known
as cotton-ball borax because it forms in soft, rounded,
white masses of loose texture. A little colemanite is found
in Siberia and Argentina. A most interesting occurrence
is in a fossil egg picked up along the Gila River in Arizona. Colemanite bears the name of a founder of the
California borax industry, William T. Coleman, of San
Francisco.
1.21
APOPHYLLITE
KEYS: Nonmetallic luster. Leaves white marie or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: Colorless. white.
Specific Gravity: 2.3-2.4 (medium weight).
~,~--- PeQrly
luster
One. cleavage
A silicate mineral containing a large amount of water
[Ca.K (Si.0 1o) 2F.8H20], apophyUitp. was named from the
fact that it unfolds like a leaf when it is heated. Its whitish-pearly look has been described as like the eye of a
boiled fish, and it once bore a much weirder name which
meant "fish eye." The natural home of apophyLlite is in
cavities in basalt and other lava rocks, in many localities
throughout the world. It is thus associated with the zeolite minerals, which it resembles in many ways. The largest and most beautiful crystals of this mineral were uncovered during the construction of a railroad at Poona,
India. Good-sized crystals stained with bitumin occur in
the New Almaden mercury mines in California. Delicate
pink specimens are found in the silver veins at Andreasberg in the Harz Mountains, Germany, and appealing
crystals of the same tint rest firmly upon amethyst at
Guana juato, Mexico. The volcanic rocks near Paterson,
N.J., near Philadelphia, Pa., and in northern Michigan
yield apophyllite.
122
WOLLASTONITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by copper coin, but can be scratched by
knife blade.
Color: White. Specific Gravity: 2.8-2.9 (medium weight).
Vitreous luster
~_
____,---:::::::=::::':7.i
Wollastonite is calcium silicate (CaSiO s)' Named in 1818
in recognition of the work of the English chemist William
Hyde Wollaston (1766'1828), its name is easy to remem·
ber when you know that it is pronounced to rhyme with
"wool" and that this mineral is used industrially as raw
material in the making of rock wool for insulation. Wollastonite is one of the interesting minerals referred to as
geologic thermometers because they indicate the temperature at which they must have originated within the rocky
crust of the earth. Wherever wollastonite is found, the
temperature there is known to have been below about
1125 cC., because above that point a different kind of
calcium silicate iii formed instead. Large white crystals of
wollastonite are common at Diana, N.Y. Showy crystals
line some of the cavities in blocks thrown out by the
volcanic explosions of Mount Vesuvius. Crestmore, Calif.,
is a noted locality for large amounts of wollastonite, as
well as for a diversity of unusual minerals scarcely equaled
anywhere else. A compact variety of wollastonite is found
in Isle Royale National Park, Mich.
EPIDOTE
Keys: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Can scratch
glass and be scratched by quartz.
Color: Yellowish-green, blackish-green.
Specific Gravity: 3.3-3.5 (medium weight).
Vitreous
Ju~ter--...
Crysta ls
than rock
One
Cleavage
Epidote, a hydrous silicate mineral [Ca 2 (AI,Fe)a (Si0 4) sO H], occurs in many localities. When found in good crystals, it is a most attractive mineral, in spite of its typical,
r .. ther monotonous yellowish-green color, which is often
described as pistachio-green and is a fairly distinctive
property for identifying epidote. This color grades into
brownish-green, gray, and black, though it may be red
or even disappear entirely in the very rare colorless specimens from remote Tierra del Fuego at the tip of South
America. Magnificent dark-green crystals, transparent
and lustrous, are present in rock cavities near Salzburg,
Austria. On Prince of Wales Island, Alaska, are found
d ark crystals of extraordinary size and beauty, which have
dominated the market in recent years. Other excellent
crystals carry labels reading Haddam, Conn., and Bourg
d'Oisans, France. Those from the Zillerthal, Austria, are
sometimes rose-red in color. Epidote is associated with
native copper in northern Michigan, as a product of alteration from other minerals. Clear epidote, when cut
into gems, is d ark-green in one d irection and brown in
another.
124
FELDSPAR
KEYS: Nonmetallic luster. leaves white mark or scratch
on strea k plate. Shows good cleavage. Can scratch
glass and be scratched by quartz.
Color: White. pink.
Specific Gravity: 2.6-2.8 (medium weight).
Blocky
cleavage
Feldspar is a group of minerals of the greatest scientific
importance. All members of the group are aluminum silicates and have varying amounts of other chemical elements, especially potassium, sodium, and calcium. The
feldspars resemble one another so closely that they usually
have to be identified with a microscope. It is customary
to divide them into two main types- plagioclase and
potash feldspar. Plagioclase is recognized by the presence
of a series of closely spaced straight lines on the cleavage
surface. This type has been arbitrarily divided into six
members, of which albite and labradorite are the best
known, but only labradorite can be named at sight, owing to its dark color and radiant blue sheen which spreads
across the surface as the specimen is turned. Of the other
type, called potash feldspar, the chief members are orthoclase and microcline. If the specimen is a clear crystal, it
is orthoclase; if green, it is sure to be microcline-the attractive variety called amazonstone, most abundant in the
Pikes Peak region of Colorado. The feldspars are the most
common of all minerals.
CORUNDUM
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by quartz.
Color: Bluish-gray, brown, pink, blue,
Specific Graviiy: 4.0 (heavy).
Corundum is aluminum oxide (AI 20 s). Its hardness, surpassing that of all other minerals except diamond, enables it to serve as an abrasive, but its real value lies in the
beauty of its colors. Few of us would think that the fiery
red of ruby and the serene blue of sapphire belong to the
same mineral, unless we were to see the original crystals.
Sapphires of yellow, green, purple, and other colors are
also corundum. The Orient-Ceylon, Kashmir, Burmais the favored home of all these gems, but they are found
in Siam, in Australia, and elsewhere as well. A prized variation of corundum is a star ruby or star sapphire. Large
rough crystals of ordinary corundum come from Steinkopf, South Africa, and from the districts of Zoutpansberg and Pietersburg, Transvaal-a jumbo-sized specimen
from Pietersburg weighed 335 pounds. Corundum is
abundant in parts of North Carolina, South Carolina,
and Georgia, and in Ontario, Canada. When it is intimately mixed with magnetite, the natural product is
called emery, a useful grinding material coming chiefly
from Turkey and islands off the coast of Greece.
126
TOPAZ
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by quartz.
Color: Colorless, yellow, bl ue.
Specific Gravity: 3.4-3.6 (medium weight).
Vitreous
luster - - - .
T opaz, an aluminum silicate with fluorine [A1 2 SiO. (F,.
OH) 2]' constitutes one of the major gem minerals. AI·
though usually thought of as a yellow stone, topaz is more
apt to occur in some other color, of which blue and pink
are certainly the loveliest. Many crystals fade upon exposure to much sunlight, and others can be altered by
heat to produce hues that are more salable. Enormous
crystals, colorless and blue, and weighing hundreds of
pounds each, have come from the state of Minas Gerais,
Brazil, in late years. Charming peach-colored crystals,
which soon lose all traces of color in the bright desert
sun, can be obtained in almost unlimited numbers in
the Thomas Range of western Utah. Large blue crystals
come from cavities on the steep slopes of Pikes Peak, Colo.
Those from Mason County, Tex., are similarly fine. The
deep-wine crystals from south of Nerchinsk, Siberia, make
outstanding specimens. The word Topazion, from which
topaz is derived, is the name of an island in the Red Sea
between Egypt and Arabia, but previously was applied to
a d ifferent mineral.
DIAMOND
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Shows good cleavage. Cannot be
scratched by quartz.
Color: Colorless, white, gray,
Specific Gravity: 3.4 (medium weight).
Pure carbon in composition (C), diamond is the hardest
of all known substances, the most popular of all gems,
and scientifically the most remarkable of all minerals.
Industrial diamonds are unexcelled for their ability to
cut anything from granite rock to other diamonds. The
word diamond comes from the Greek language and means
invincible, in reference to this extraordinary power. With
their sensitive cleavage, however, gem diamonds cannot
be subjected to sudden blows, in spite of their resistance
to scratching-this fact is the basis for the convenient
method used to cleave diamond crystals in order to reo
move flaws and prepare the stones for faceting. India was
long the principal source of diamond and furnished most
of the famous stones of history. In the eighteenth century
it was replaced by Brazil as the leading producer, a position assumed by South Africa in about 1870' The Belgian Congo is today the chief supplier of industrial diamond. A considerable yield of diamond has come from
Pike County, Ark., in currently unproductive fields.
uS
SULFUR
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Can be
scratched by fingernail.
Color: Yellow. Fracture: Even.
Specific Gravity: 2. 1 (light).
Known to the ancients as brimstone, native sulfur (S) is
a mineral of widespread occurrence. To the mineral collector, any of the bright yellow masses are worth having,
but the crystals themselves are indeed intriguing. When
they are held close to the ear, they can be heard to crackle,
on account of the warmth of your hand, because the outer
layers expand away from the still-cool interior. Obviously
a crystal of sulfur should be handled carefully. The large
crystals from Girgenti, Sicily, and nearby are the most
beautiful known. Good crystals are found with asphalt at
Perticara, Italy. Attractive porous specimens come from
Sulphur, Nev. The vast deposits of Louisiana and Texas,
which lie above the so-called salt domes and are "mined"
by melting the sulfur with superheated steam, furnish
most of the world's needs. Rocks in the vicinity of practically every active volcano are coated with su!fur. Orpiment may resemble sulfur, but it has a pronounced cleavage. Sulfur burns easily with a bluish flame, giving off
the pungent odor of sulfur dioxide.
12 9
BAUXITE
KEYS: Nonmet allic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Can be
scratched by finge rnail.
, gray, brown. Fracture: Uneven.
~~~)'E!Ci~t:it1WJ_iity:
2.0-2.6 (medi um weight) •
.... .. '.
Bauxite has been proved to be a mixture of a number of
water-containing aluminum oxide minerals, and not a
single mineral as was formerly believed. The constituents
are shown by X rays to be mostly gibbsite, boehmite, and
diaspore. Still, the name has come into such general use
for a highly important commercial substance, and bauxite is usually so uniformly easy to recognize, that we are
accustomed to regard it as a minmal instead of a rocklike
mixture. The nobby structure IS almost always present,
the rounded grains varying in size and color; iron is the
staining matter. Bauxite originates from the weathering
of aluminum.bearing rocks in a warm climate. First dIScovered at Les Baux, France, it is produced in enormou:.
quantities in Dutch Guiana, British Guiana, and Arkansas and in the Mediterranean countries of Southern
Europe. Whether you prefer to think of bauxite as a mineral or a rock, Its significance cannot be denied. From it
comes most of our supply of the light, strong, cond uctive,
and resistant wonder metal of the twen tieth centuryaluminum.
13°
KAOLINITE
KEYS : Nonmetallic luster. leaves white mark or scratch
on streak plate. Does not show good cleavage. Can be
scratched by fingernail .
Color: White. Fracture: Eorthy.
Specific Gravity: 2.6 (mediu m weight).
Clay_,..
The best known of the many clay minerals, kaolinite is.
like the rest of them, a h ydrous aluminum silicate [AI 2Si 20~ (OH) . ]. Other species of clay minerals related to
kaolinite include dickite, nacrite, beidellite, montmorillonite, and halloysite. An impure mixture of various of
these minerals- often called simply kaolin-is common
clay, which is the basis of the porcelain, pottery, and
china industries. Both names are a corruptIOn of the
Chinese name for a hill near Juachu Fa, where the finequality clay was produced from which china was made.
Kaolinite is formed by the weathering of rocks that contain a large proportion of feldspar; until the alteration is
complete, particles of feldspar are found mixed in the soil
as it is being created. The thin crystal plates of kaolinite
are seldom large enough to be seen without a microscope;
they tend to curl up like dry mud. Masses of kaolinite
can often be recognized by a peculiar earthy odor when
they are breathed upon, and they become plastic when
moistened, adhering slightly to the tongue.
VANADINITE
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratched by fingernail, but can be scratched by
copper coin.
Color: Orange. Fracture: Uneven.
Specific Gravity: 6.7-7.1 (very heavy).
AdamQnhne
ILAster--+
ften hollow
In composition a vanadate of lead and chlorine (PbsCl.
(VO.) B). vanadinite is an ore of both vanadium and
Jead. It occurs witb other lead minerals. though never in
quantity. The most interesting feature ot this mineral is
its curious crystals. Often they grow in barrel-shaped
forms. hollow inside. When straight, they have smooth
faces and sharp edges and resemble battlements of a six·
sided castle. They look a good deal like crystals of pyromorphite and mimetite, unless their color happens to be
quite reddish. Arsenic is a frequent impurity; as it in·
creases, vanadinite becomes the light-yellow variety formerly called endlicbite. Lustrous yellow and red crystals
of vanadinite have been found in Arizona and New Mexico. Those from the Old Yuma mioe in Pima County and
the Red Cloud mine in Yuma County, both in Arizona,
are truly thrilling to see. Zimapan, Mexico, was the first
known locality. Large crystals have come from Djebe1
Mahseur, Morocco, and fine ones also from Grootfontein,
South-West Africa.
13.2
SERPENTINE
KEYS: Nonmet allic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratc~ed by fingernail, but can be scratched by
copper com.
Color: Green. Fracture: Uneven.
Specific Gravity: 2.5-2.8 (medium weight).
Darker
mottlinq
Serpentine, hydrous magnesium silicate [MgsSi20s (OH).],
owes its name to the snakelike pattern of the variegated patches of darker and lighter color that are so typical of it. This mineral may be either platy or fibrous. The
platy type is termed antigorite. When fibrous, the variety
is called chrysotile, which may have the fibers so well developed as to become true asbestos, although this is not
the only asbestos mineral. Thetford, Que., is the asbestosmining capital of the world. The Canadian deposits extend across the border into New York and Vermont.
Among other places, some is even found in the Grand
Canyon. Crystals of serpentine are completely unknown,
except when they have resulted from the alteration of
some older mineral, as, for instance, at the Tilly Foster
mine, near Brewster, N.Y. Masses of serpentine have long
been used for decorative purposes and as a building
stone; a mixture with white carbonate minerals is called
verd antique. Serpentine looks like green marble, which,
in fact, is often almost solid serpentine.
CERUSSITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratched by fingernail . but can be scrat ched by
copper coin.
Color: White, gray. Fracture: Conchoidal.
Specific Gravity: 6.4-6.6 (very heavy).
A lead carbonate (PbCO a). cerussite is an important ore
of lead. and sometimes it contains silver which was present in the original mineral. galena. from which it alters.
Anglesite. the lead sulfate. sometimes represents an intermediate stage. The Friedrichssegen mine near Ems. Germany, gave us the first known specimens of cerussite.
Large crystals ot several colors. even including green. have
been found at Nerchinsk. Siberia. Groups of crystals
shaped like arrowheads come from Broken Hill. New
South Wales. Incomparable specimens occur at Tsumeb,
South-West Africa. Heart-shaped twins of large size have
been collected in Dona Ana County, N.M. Cerussite is
abundant at Leadville. Colo., and in the Coeur d'Alene
district, Idaho. Its name is derived from the Latin word
meaning white lead, an artificial product known by 400
B.C. or earlier; natural cerussite is sometimes called white
lead ore. As can be said of all carbonate minerals, cerussite fizzes in acid. but satisfactory results can be had only
with nitric acid, which is required to dissolve it.
134
BORAX
KEYS: Nonmetallic luster. Leaves white mark or scratch
on strea k plate. Does not show good cleavage. Cannot
be scratc~ ed by fingernail, but can be scratched by
copper cOin.
Color: White, colorless. Fracture: Even.
Specific Gravity: 1.7 (light).
Vit reous
luster - -....
A hydrous sodium borate (Na 2 B.O r . loH 2 0), borax is a
mineral having an old Arabic name. In 1856 it had been
discovered at Borax Lake, Calif., and large crystals were
removed from the mud at the bottom of the lake. Later,
borax supplied the famous Death Valley twenty-mule
teams with the useful product also called borax, which is
employed in a hundred industries. From Furnace Creek
and Resting Springs they hauled it to the railroad at
Mojave. Searles Lake is now the world's largest deposit,
one of astonishing volume. Around the borax lakes in
California, and similarly in Tibet, the mineral is found
to have crystalhzed on the shore. Some of the American
deposits are situated in Nevada. Borax is also obtained
from hot springs and brines in northern Italy. The dyedin-the-wool mineral enthusiast who enjoys licking samples
to see them shine would find that borax has a slightly
sweetish yet alkaline taste. It is soluble in water and turns
to chalky whi le tincalconite in a dry atmosphere.
APATITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratched by copper coin, but can be scratched by
knife blade.
Color: Green , brown. Fracture: Even.
Specific Gravity: 3.1-3.2 (medium weight).
Vitreous
luster ----',..,
Apatite is calcium phosphate with the formula Ca6(PO. ) 8 (F,CI,OH). This intriguing name has nothing to
do with food but comes from a Greek word meaning "to
deceive," because of the resemblance to other minerals.
Still, apatite does relate to food, in that it is a constituent
of tooth enamel. No other crystals of apatite have ever exceeded in quality those found at Knappenwand in Austria, where they are complexly formed and wonderfully
limpid. The largest apatite deposit is situated on the
Kola Peninsula in the Soviet Union, where it is mined for
fertilizer on account of its phosphate content. Substantial
amounts occur along the southern coast of Norway. The
clear greenish-yeJ1ow crystals from Mexico are appropriately called asparagus stone. The many excellent specimens from New England include purple crystals from
Maine and dark-greenish and blue ones from Connecticut. A single crystal weighing over 550 pounds was found
in Buckingham Township, Que. The luster of apatite has
almost a resinous look.
136
PYROMORPHITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratched by copper coin, but can be scratched by
knife blade.
Color: Green. Fracture: Even.
Specific Gravity: 6.5-7.1 (very heavy).
ResinolJs luster__.
Often hollow
Usually - - - - +
barrel-shaped
Phosphate of lead and chlorine [Pb 5CI (PO.) 3], pyromorphite is the most prominent member of a series of minerals which closely resemble one another in composition.
With a substitution of arsenic for phosphorus, pyromorphite grades into mimetite; the crystals of both are similar to those of vanadinite, and all these minerals have a
fairly wide range of shades besides the most typical colors
stated here as their properties. Not only do the crystals of
pyromorphite occur in hollow barrel-shaped forms, but
they also grow in parallel six-sided tubes which expand
upward from a slender point. Some of the finest crystals
are the violet and brown ones from Poullaouen and Huelgoat, France. Good specimens have come from Phoenixville, Pa.; Friedrichssegen, Germany; and Horcajo, Spain.
Pyromorphite is a lesser ore of lead, and it occurs with
other lead minerals in the upper levels of mines. The
name is derived from Greek words meaning "fire" and
"form," because a melted drop acquires a crystalline
shape when it cools.
WILLEMITE
KEYS: Nonmetallic luster. Leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratched by copper coin, but can be scratched by
knife blade.
Color: Yellowish-green, brown. Fracture: Uneven.
Specific Gravity: 3.9-4.2 (hellvy).
Willemite, zinc silicate (Zn 2 SiO.), is a valuable ore of
zinc at Franklin, N.j., where its intimate association
with black franklinite, red zincite, and white calcite
makes one of the most distinctive combinations of minerals known anywhere. Willemite is white when pure, but
its normal colors are yellowish-green and reddish·brown.
Slender crystals of a delicate apple-green color, and larger
flesh-red crystals containing manganese and known as
troostite also occur in New Jersey. The glowing fluorescence of two of the minerals from this extraordinary 10·
cality-willemite appearing green and calcite rose-redconstitutes a striking feature of the mineral a~semb J age.
Because of it the willemite can be hand-picked from conveyer belts moving under an ultraviolet light. Willemite
has also been found in a few isolated locali ties in the
western part of the Uni ted States; at Alten berg, Belgium;
Musartut, Green land; and several places in Africa. I t was
n amed after King W illiam I of the Netherlands (Willem
Frederik, who reigned from 1815 to 1840) .
13 8
WULFENITE
KEYS: Nonmetallic luster. leaves white mark or scratch
on streak plate. Does not show good cleavage. Cannot
be scratched by copper coin, but can be scratched by
~nife blade.
Color: Yellow. Fracture: Even.
Specific Gravity: 6.3-7 .0 (very heavy).
Wulfenite, which is a lead molybdate (PbMoO.), is only
to a small extent an ore of molybdenum, but its crystals
are some of the loveliest in the mineral kingdom. Some of
them, resembling wafers of butterscotch candy, look tasty
enough to be eaten. The orange·red tabular crystals from
the Hamburg and Red Cloud mines in Yuma County,
Ariz., are as much as 2 inches long. Paper-thin crystals of
good size come from Box Elder County, Utah; and a
dozen other localities in the Western states of Nevada,
New Mexico, Arizona, and Utah have produced superior
crystals grouped at various angles like the disarranged
compartmen t of a candy box. The color runs from a brilliant red to orange, yellow, and brown and at times may
be green or gray. Specimens from the Sierra de los Lamentos, Mexico, grace many a collector's cabinet. This
mineral was named in 1845 for Franz Xaver von Wulfen
(1728-18°5) , an Austrian Jesuit priest and mineralogist.
It usually resu lts from the decomposition of other min·
er~ls. I ts associates are vanadinite and pyromorphi te.
139
SPHENE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by copper coin, but can be scratched by knife
blade.
Color: Greenish-yellow, brown. Fracture: Conchoidol.
Specific Gravity: 3.4-3.5 (medium weight).
Concnoidal
'-'fracture
A silicate of calcium and titan ium (CaTiSiO ~), sphene
owes its name to the wedge-shaped crystals in which it
often grows. An equally familiar name is titanite, indicating its chemIcal composition. Large crystals, darker in
color than average, are found in a number of places in
New York, Ontario, and Quebec. Those from Diana,
N.Y., and Eganville, Ont., are dark-brown; those from
Litchfield, Que., are shiny-black. Some of the broad reddish crystals from the Ala Valley in Italy are exceptionally attractive. Numerous places in Switzerland yield
good crystals of sphene, including the large pale-green
twins occurring at Saint Cotthard. The Kola Peninsula
of Russia, between the Arctic Ocean and the White Sea,
contains the largest deposit, used there as an ore of titanium. Transparent sphene, though not overly abundant, makes spectacular gems because of its superior luster
and light-refracting power, but its softness hinders its usein jewelry. Sphene is common in granite but as an accessory to the more essential minerals.
140
SCHEELITE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by copper coin, but can be scratched by knife
blade.
Color: W hite. gray. Fracture: Even.
Specific Gravity: 5.9- 6.1 (heavy).
Calcium tungstate (CaWO.) is scheelite. Although other
ores yield more of the world's supply of tungsten, scheelite is the most important American source. It is one of
the few minerals that can almost always be relied upon to
fluoresce in ultraviolet light, the ensuing color being a
bright bluish-white. As molybdenum replaces some of the
atoms of tungsten, scheelite grades toward another mineral, powellite, and the fluorescence becomes white and
then yellowish. Mineral collectors using short-wave ultraviolet lamps have come across numerous small occur·
rences of both minerals during the past decade. Scheelite
was named in honor of Karl Wilhelm Scheele (17421786) , the Swedish chemist who discovered in it the element tungsten, now called wolfram. Crystals of scheelite
are not common; Traversella, Italy, and Framont, France,
are noteworthy localities for them, and some of consider·
able size have been found in the tin mines of Germany,
Czechoslovakia, and England. Large deposits of scheelite
occur near Mill City, Nev., and Bishop and Atalia, Calif.
141
DATOLITE
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by copper coin, but can be scratched by knife
blade.
Color: Colorless, white, light-g reen.
Fracture: Conchoidal.
Specific Gravity: 2.9-3.0 (medium weight).
Conchoidal fradl.lre
A basic silicate of calcium and boron [CaBSiO. (OH) ],
datolite is a mineral of secondary origin, forming in cavities in lava rock, such as basalt. It is associated with the
zeolite minerals. Its occurrences are in regions where
volcanic rock is abundant; for instance, in New Jersey
and the Connecticut River Valley in the United States,
and in the Harz Mountains of Germany. Datolite is one
of the characteristic minerals in the Michigan copper deposits around Lake Superior, where, in addition to fine
crystals having a greenish tinge, cream-colored and pinkish porcelainlike masses are found which contain inclusions of native copper and are sometimes polished as gem
material. Italy and Norway are other leading countries
for specimens of datolite. The complex crystals, usually of
pale-green tint, have many faces distributed at odd angles
over the surface. Otherwise this mineral is difficult to
recognize without chemical tests.
142
ARAGONITE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by copper coin, but can be scratched by knife
blade.
Color: Colorless. white. groy. Fra cture: Conchoidal.
Specifi c Gravity: 2.9 (medium weight).
Vitreous I lA st~ r~r-El;''''''''~\J
Moy show ~
bl ock y cleavage
+--Conchoidol
fracture
Aragonite is calcium carbonate (CaCO s ). It thus has the
same chemical composition as calcite, and, like all carbonates, i. fizzes in acid. Although less common than calcite, aragonite is formed under various conditions
ranging from the bot water of geysers to the cold water
of the ocean. It also forms as a crust inside of teakettles
and water tanks. Named in 1796 from its original locality
in the former kingdom of Arag6n, Spain, this interesting
mineral has been found in almost every country. Some
collectors like best tbe six-sided disks known in Wyoming
and Colorado as "Indian dollars" and in New Mexico as
"Aztec money: ' Some of these are twin crystals which
have changed over to calcite. Other collectors prefer the
twisting branches tbat resemble snow-white coral and
are known as "f1os ferri." Large, well-formed crystals of
aragonite occur in the sulfur deposits on the island of
Sicily. The upper part of many coral reefs in the Pacific
O cean consists of aragonite, as does the mother-or-pearl
of oyster and abalone shells.
ANALCIME
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by copper coin, but can be scratched by knife
blade.
Color: Colorless, white. Fracture: Even.
Specific Gravity: 2.2- 2.4 (medium weight).
Analcime is a hydrous silicate of sodium and aluminum
(NaAISi0 6 .H2 0). It is one of the most important members of the zeolite family, all of which boil up, giving off
water evenly when heated. The word zeolite, in fact,
comes from the Greek word meaning "to boil." The beautiful glassy crystals of analcime in the Cyclopean Islands
in the Mediterranean Sea are typical of how this mineral
occurs in cavities in volcanic rock. Large white crystals
are found on the Seiser Alp, Italy. Some giant crystals
of analcime measure a foot in diameter. Among many
other localities are those on isolated Kerguelen Island in
the Indian Ocean ; at Cape Blomidon, Nova Scotia;
North Table Mountain, overlooking Golden, Colo.;
Bergen Hill, N.].; and in the Lake Superior copper district. Analcime, also written analcite, has the same appearance as leucite and white garnet; unlike leucite, it
grows in rock cavities, and it is softer than garnet. Analcime was named in 1797 from the Greek word for "weak"
because of its feeble electrical nature.
144
MAGNESITE
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by copper coin, but can be scratched by knife
blade.
Color: Green. Fraclure: Conchoidal.
~pecif\c Gra'Vit-t~ 2.<1-3. \ ~medium weigt.'\.
Vitreous luster
Porceloinlike
Often
+-rhombic
cleavaqe
As its name suggests, magnesite is magnesium carbonate
(MgCO a). It, like the rest of the carbonates, fizzes in
acid, which should be warmed for best results. Magnesite
is used to make bricks for lining metallurgical furnaces,
and it has served as an ore of magnesium metal, now
extracted mostly from sea water. Large clear crystals occur at Born Jesus dos Meiras, Brazil. Some of the rare
prismatic crystals are found at Orangedale, Nova Scotia.
Sizable deposits of commercial magnesite are situated at
Chewelah, Wash.; bordering both the Coast Ranges and
the Sierra Nevada in California; and in the Paradise
Range in Nevada. Abroad they are especially large in
Manchuria, the Ural Mountains, and Austria. A classic
locality is the island of Euboea, Greece. Magnesite is
slightly harder than calcite, and when occurring in cleavable masses it may require a chemical test for magnesium
to distinguish it from calcite. At times the usual porcelainlike compact varieties give the impression of being
harder and tougher than they are.
OLIVINE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: Green. Fracture: Conchoidal.
Specific Gravity: 3.3-3.4 (medium weight).
.
.
a:r~~~}
~.r~ _>' ._~ _~ ll .,....~ .
VItreous luster--...);' .r-~-:: ". ,,''; .- :J
rJ"
• Y ... · «-....l
. ;J \ .. " ',~';:,J )
t~l"" ..04 => "~f..ll., \.:- " '; ...~ - •. ...J. .. '
) I'
Granular.
suqary texture
~ .;::~,.l.
_;?.:;, "",J~'~
---~~ " '~ ~_.<,"::-.~ . ~3
f~ I _:' .-- "'" ;"
~onchoidol
>.
~
frQcture
This magnesium-iron silicate [(Mg,FehSi04] was so
named because of its olive-green color. With its tendency
to form separate grains, a specimen may resemble somewhat a bowl of nearly round pale olives much shrunken
in size. Olivine is a member of a mineral series grading
from forsterite to fayalite. When transparent and having
a distinctive bottle-green shade, olivine is fashioned into
gems called peridot. St. John's Island in the Red Sea is
the most celebrated locality for peridot, and rounded
grains may be secured on some of the Indian reservations
in the American Southwest. Common olivine is abundant
in many dark and heavy rocks, those formed deep in the
earth's crust. The rocks called dunite and peridotite are
composed entirely or mostly of olivine. Volcanic bombs
are often solid masses of olivine, and crystals of olivine
are found in the lava which has flowed from Mount
Vesuvius. Olivine is the most typical mineral in nonmetallic meteorites. The largest known crystals, from
Snarum, Norway, have almost completely altered to
serpentine.
146
IDOCRASE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: Green, brown. Fracture: Uneven.
Specific Gravity: 3.4 (medium weight).
Stria tions
A complex silicate of aluminum, idocrase is also known
by its older name, vesuvianite, which indicates one of its
first known localities, Mount Vesuvius, where sparkling
brown crystals occur. A compact green variety with whitish streaks, resembling jade, is called californite; it is
found in Siskiyou, Fresno, and Tulare Counties, Calif.
Other localities in the United States include those situated in Arkansas, Maine, Vermont, New Jersey, and New
York. In Quebec large brownish-yellow crystals of idocrase occur at Calumet Falls, while at Templeton the
crystals are brownish-red. The clearest and best-formed
crystals are the transparent green and brown ones from
Ala, Italy. Large brown crystals come from Egg, near
Kristiansand, Norway. Idocrase originates as a result of
the action of heated rocks on beds of limestone, which
may be changed to marble with segregations of idocrase
and other minerals, such as garnet and diopside. Besides
the californite variety, clear specimens of idocrase have
been used as gem stones; a superior locality for them is
Eden, Vt.
147
TURQUOISE
KEYS: Nonmetallic luster. leaves white marlt or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: Blue, green. Fracture: Conchoidlli.
Specific Gravity: 2.6-2.8 (medium weight).
veininq
Turquoise, a hydrous aluminum phosphate [CuAl,(PO.) • (OH) 8.4H20], is one of the minerals most favored as a gem since prehistoric times. Ancient mines in
Egypt and Persia produced it many centuries ago, and
the Persian deposits near Nishapur are still productive.
The material they yield is accepted as the standard of
quality for clear blue stones containing a minimum of
iron-colored matrix. When the matrix, or ad joining rock,
is present as a delicate veining, it creates the delightful
spider-web pattern, desired by many buyers as a sign of
genuineness. Turquoise reaches its peak of popularity
with the American Indian, to whom it holds a good deal
of symbolism as well as beauty. The once extensive
deposits in New Mexico are largely depleted, the leading
states now being Nevada and California. The mineral
occurs in stringers and small nodules. The largest nugget
of turquoise on record, weighing almost 9 pounds, was
mined in the King mine near Manassa, Colo., in 1945.
The word turquoise is French for "Turkish," the Persian
stones having reached Europe by way of Turkey.
148
PREHNITE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: Light-g reen. Fracture: Uneven.
Specific Gravity: 2.8-3.0 (medium weight).
Radial structure
I
A hydrous aluminum silicate [Ca2Al2SisOlo (OHh].
prehnite was named in 1790 in recognition of Col. van
Prehn, who brought the first specimen to Europe from
South Africa. It resembles the zeolite minerals in some
respects and occurs with them in cavities in volcanic
rocks. First·rate crystals are found at Coopersburg, Pa.,
but elsewhere prehnite grows mostly in jug-shaped or
rounded aggregates, in which the individual rectangular
plates of the cockscomb structure are scarcely noticeable
except upon close examination. Prehnite looks like hemi·
morphite, which is, however, far more resistant to heat;
although prehnite fuses easily, it does 1.ot give off water
quite so readily. Interesting barrel-shaped groups, enclosed in white asbestos, have been found near Bourg
d'Oisans, France. Leading localities in America for
prehnite are Farmington, Conn.; Somerville, Mass.; the
New Jersey trap quarries; and the Michigan copper
region. Localities in other countries include the Kilpatrick Hills in Scotland, and Cradock, Union of South
Africa, from where the original material came.
149
LEUCITE
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: White, gray. Fracture: Conchoidal.
Specific Gravity: 2.5 (medium weight).
Embedded
in rock
Conchoidal
fracture
Leucite is a potassium-aluminum silicate (KAlSi 2 0 e)
and was named because of its color; it is often called
white garnet, which it resembles. Like sodalite, leucite
belongs to the feldspathoid group of minerals. It originates at high temperatures in lava rocks. The best crystals
are from Mount Vesuvius, where small clear crystals fall
like hailstones during eruptions and large perfect crystals are found in the big blocks of rock thrown out by
the violence of the explosion. Magnificent groups of
leucite crystals have been picked up as drift boulders on
the shore of Vancouver Island, B.C. The Leucite Hills
in Wyoming contain this mineral in abundance, and so
do some of the mountains in Montana-the Bear Paw
and Highwood Mountains, for example-but otherwise
leucite is rare in the United States. It can be distinguished from garnet by its being softer and from analcime, another similar-looking mineral, by its favoring
solid rock in which to grow, rather than cavities. Leucite
is not too stable a mineral, converting to white or gray
alteration products from its naturally colorless state.
150
QUARTZ
KEYS : Nonmet allic lust er. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: Colorless, white, groy, black.
Fracture: Conchoidal.
Specific G ravity: 2.7 (mediu m weight).
Often contains
inclusions____.
0
, ::
-
Conchoidal fracture
Although an oxide of silicon (Si0 2 ) in chemical composition, quartz is now considered a sort of parent of the
enormous number of silicate minerals, inasmuch as its
atomic structure is similar to them. So numerous are the
varieties of quartz that they occur in every color and a
fantastic array of patterns. Rock crystal-such as fortunetellers use for spheres, scientists usc for controlling frequency in electronic devices, and the rest of us use for
jewelry-is colorless quartz. Smoky quartz, as from the
Alps, is richly brown or hazily black. Cairngorm, the
national gem of Scotland, looks as topaz is popularly supposed to look. Rose quartz has a pink color, generally not
lasting. Tiger's-eye, from Griqualand West, South Africa,
is a blue asbestos turned to golden quartz. The most desired kind of quartz is amethyst, its choicest crystals a
delectable purple or violet. The gem varieties of quartz
are perhaps found most conspicuously in Brazil, Madagascar, and the United States.
CHALCEDONY
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glass and be scratched by quartz.
Color: Brown, green, gray, black, white.
Fracture: Conchoidal.
Specific Gravity: 2.6 (medium weight).
Vitreous
luster--~
Often
banded
The atomic structure of certain varieties of quartz is so
minutely developed that they are never able to mature as
actual crystals with definite outward shapes. Many min·
eralogists classify them as belonging to a different mineral. called chalcedony. Some of the kinds of chalcedony
are fibrous. while others are granular, but names are applied to them according to their colors and patterns, of
which there are many. Agate, for instance, is a banded
chalcedony. though moss agate reveals fernlike designs.
Onyx is likewise banded, the layers being straight. Carnelian is red chalcedony; sard is brown; chrysoprase is
green; bloodstone is also green but with red spots resembling drops of blood. Jasper occurs in many colors, typically dark-red, green, or yellowish-brown. Flint is the gray
or black chalcedony used for implements by primitive
man. Every part of the world furnishes these varieties for
gem use, and they are the favorite stones of the thousands
'01 amateur lapidaries who have a rewarding hobby in
cutting and polishing them.
15 2
O'''l
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Can scratch
glas$ and be scratched by quartz.
Color: White. Fracture: Conchoidal.
Specific Gravity: 1.9-2.3 (light).
Conchoidal
frQctlJre~
When water is present in a chalcedony-like substance, the
material is hydrous silica (Si0 2 .nH 2 0). called opal. This
is one of the few minerals not having a definite atomic
structure and never occurring in crystal form. Its varying
appearance seems to reflect this inconstancy. Many lovers of fine gems regard opal as their favorite. Lacking
any color of its own except as a background. it reveals
all the hues of the rainbow in ever-changing. purest aspect. White opal. also called Hungarian opal. has a pale
or white background. Black opal has a background which
is usually dark-blue or gray. against which the colors
flash. Australia is the true home of black opal, first discovered in 1905 at Lightning Ridge, New South Wales.
Some of it is also produced from time to time in Humboldt County, Nev., but is apt to crack after exposure.
The reddish fire opal from Queretaro, Mexico, is occasionally quite attractive. Mineral collectors, however, are
likely to find only the so-called common opal, which does
not show the distinctive play of colors. Opalized wood is
abundant in western United States.
TOURMALINE
KEYS: Nonmetallic luster. leaves white mar't or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by quam.
Color: Block , pink, green. Fracture: Conchoidal.
Specific Gravity: 2.9-3.2 (medium weight).
-
,Striations
Rounded-
-
triangular
Tourmaline is an aluminum silicate with a formula so
complex that John Ruskin said, "T he chemistry of it is
more like a medieval doctor's prescription than the making of a respectable mineral." It occurs in a tremendously wide range of colors. More than that, the colors are
often zones either along the length or across the width of
the unique rounded-triangular crystals. Watermelon
tourmaline, for example, has a green exterior, surrounding first a white zone and then a red core. Each of the
colors of tourmaline has a different name-rubellite, indieolite, achroite, dravite are some of them-but they are still
tourmaline and eminently suited for use as gems. The jetblack specimens, which resemble hornblende, tend to
fracture like coal. Pierrepont Manor, N.Y., is a renowned
locality for black tourmaline. The more colorful varieties
are especially noteworthy in the United States at Pala,
Calif., and at Auburn and Paris, Me. The most prominent
localities abroad are situated in Brazil, the Ural Mountains, and the islands of Ceylon, Madagascar, and Elba.
154
STAUROLITE
Kt'l'S: Nonmeta\\ic \ustef. leaves white mar\. Of s~ratcn on
streak plate. Does not show good cleavage. Cannot be
scratched by quartz.
Color: Brown. Fracture. Conchoidal.
, Specific Gravity: 3.7 Imedium weight).
Resinous
.uster--_\
+- Often shows
only one section
Conchoidal
fracture - - - - " .
Staurolite is an iron-aluminum silicate [Fe (OH)AI.(AISi 2 ) 0 12 ], Although twin crystals have already been
described for a number of minerals, staurolite is the one
mineral above all others whose twins are of outstanding
interest. Penetrating each other to form a Greek cross, to
which staurolite owes its name, they are widely collected
and kept as amulets. Called fairy crosses and figuring in
10caJ legends, those from Georgia and Virginia are eagerly sought, some being almost as smooth as though they
had been polished. Rougher ones come from Cherokee
County, N.C., and near Taos, N .M. The original localities for these crosses are in western France; large ones
come from there and from Scotland. When the twin crystals of staurolite do not make a cross, they are shaped
like the letter X instead. Single brown crystals of staurolite- lustrous and translucent- are found, on Monte
Campione. Switzerland. Fine crystals occur at Chesterfield. Mass. The untwinned crystals bear a resemblance
to andalusite, which, however, is nearly square in outline.
155
ZIRCON
KEYS: Nonmetallic luster. leaves white mar~ or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by quam.
Color: Brown . Fracture: Uneven.
Specific Gravity: 4.7 (heavy).
Adamantine luster
This is a silicate of the metal zirconium (ZrSiO.). The
name zircon has an obscure origin dating back many centuries. The metal was named from the mineral, which
has been known since ancient times as one of the more
remarkable of the gem stones. Three structural types have
been proved to exist; these are termed high, intermedia te,
and low zircon. Several beautiful colors are available,
including blue and golden yellow, as well as colorless
stones which resemble diamonds. Those that are some
shade of orange are called hyacinth and jacinth. These
have almost all been heat-treated to produce the desired
hue. Deep-red and mysterious green zircons are entirely
natural. The gemmy crystals are found mostly in Ceylon
and Indo-China. In Canada large crystals of ordinary
zircon come from Sebastopol, Ont., and Templeton
Township, Que. In Madagascar they are numerous on
Mount Ampanobe. Exquisite tiny zircon crystals accumulate in beach sands in North Carolina, Florida, and elsewhere, for they have washed out of granite and similar
rocks, in which they are a frequent constituent.
156
ANDAlUSITE
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by quartz.
Color: Gray, brown . Fracture: Uneven.
Specific Gravity: 3. 1-3.2 (medium weight).
Vit reous lust er
Square
~
Often has
mica scales
Andalusite (named after the province of Andalusia in
Spain) is an aluminum silicate with the same chemical
formula as kyanite and sillimanite (A12SiO ~ ) . When
heated, it becomes mullite, which is the porcelain used
in making spark plugs. The variety known as chiastolite
has inside a dark cross of organic matter, which assumes
different shapes as the crystal is sliced-sometimes appearing as a wedge, sometimes as an hourglass. The large
specimens of chiastolite from Rohan. France. are outstanding. as are those from the Lisenser Alp in the Austrian Tirol. Superior crystals are also found at Bimbourie,
South Australia. Gem-quality andalusite is not common
and is remarkable for the fact that the cut stones appear
either olive-green or blood-red, according to how they
are held. Stream-worn pebbles of this quality come from
Brazil. The mineral is mined in ccmmercial qua ntities
near Laws, Calif. Good specimens come from Maine,
Pennsylvania, and several places in Massachusetts; the
stou t crystals often project as kn ots from the rock. Andalusite-rich sand occurs in South Africa.
GARNET
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by q uam.
Color: Red, brown, yellow, green, blllck, white.
Fracture: Even. Specific G ravity: 3.5-4.3 (heavy).
Vitreous ·Iuster· - - - - .
Garnet is a group of minerals which are silicates of aluminum, calcium, and other elements. The various garnets
closely resemble one another, and even their colors
usually give no assurance as to their individual identity.
They are often recognized most readily by the other
minerals associated with them, because each of the different garnets has its own type of geologic occurrence.
Almandite, as mined in heavy crystals in eastern New
York, is an important abrasive. When transparent and
deep-red, this kind of garnet is known as precious garnet.
Pyrope, the familiar dark-red Bohemian garnet in grandmother's jewelry, is also called precious garnet. Rhodolite
is a lovely rose-colored garnet from North Carolina, corresponding to two parts of pyrope and one of almandite.
Spessartite is brown or red and occurs in light-colored
igneous rock. Grossularite includes white crystals, as well
as those of more usual colors. Uvarovite is always green.
Unsurpassed among all the gem garnets, however, is
demantoid, a brilliant green variety of andradite from
the Ural Mountains, used as a substitute for emerald.
IS8
BERYL
KEYS: Nonmetallic luster. Leaves white mark or scratch on
streak plat e. Does not show good cleavage. Cannot be
scratched by quartz.
Color: Bluish-green, light-yellow, white.
Fracture: Conchoidal.
Specific Gravity: 2.6-2.8 (mediu m weight).
Often shows
~ str iations
Conchoidal
f racture - - -.... 1
Beryl is a silicate of two light metals, beryllium and
aluminum [Be~AI2 (SiO s) 6]. From it is extracted the
strategic metal beryllium, which was named after it.
Beryl, furthermore, is one of the major gem stones. When
green, it is emerald ; when blue or bluish-green, it is aquamarine; when pink, it is morganite-all gems of exceptional loveliness. The emeralds from Muzo, Colombia.
have been the model of quality since the middle of the
sixteenth century. The other beryl gems are obtained
most abundantly in Brazil, Madagascar, Siberia, Ceylon,
and California. Pretty aquamarine crystals are found at
the very summit of 14,245-foot Mount Antero, Colo., the
highest mineral locality in North America. Common
beryl, used industrially. may occur in huge rough crystals. such as the two weighing 25 and 18 tons which were
uncovered at Albany, Me .• and the 40-ton monster reported from Madagascar. Another, with a weight of 3 .000
pounds, carne from Grafton, N.H. Aquamarine crystals
often weigh several hundred pounds.
159
SPINEL
KEYS: Nonmetallic luster. leaves white mark or scratch on
streak plate. Does not show good cleavage. Cannot be
scratched by quartz.
Color: Black, brown, green, pink. Fracture: Conchoidal.
Specific Gravity: 3.6 (medium weight).
Magnesium aluminate (MgA120.) in natural form is
spinel. An anciently known mineral, carrymg a name
whose origin was lost in the distant past, spinel furnishes
gems in a conSIderable range of colors which resemble
those of other gems, especially ruby and sapphire. The
Black Prince's ruby, one of the most historic stones
among the British crown jewels, has been proved to be
actually spinel. Owing to its hardness, spinel is iound as
rolled pebbles in placer sand in the gem fields of Ceylon
and Burma, together with the corundum which looks so
much like it. Crystals of ordinary spinel, colored green,
brown, and black, are distributed in a belt of rock ex·
tending from Andover, N.J., to Amity, N.Y.; and including a number of established collecting localities.
Large crystals are found at Ambatomainty, Madagascar.
Spinel crystals are like those of diamond and magnetite
but are not so hard as diamond and are not magnetic like
magnetite. Gahnite, often called zinc spinel, is a similar
mineral found in many places; crystals 5 inches across
have been acquired at Franklin, N.J.
160
CHAPTER 5
Four Keys to Recognizing Rocks
ROCK KEY NO. 1
TEXTURE AND STRUCTURE
The size, shape, and pattern of the mineral grains in a
rock are included in the term texture . The larger features
of rocks, as seen in the field, are called structures, but in
specimens of the size that can be conveniently held in
one's hand there is scarcely any difference between texture and structure as they are often observed.
Nothing indicates better than its texture the conditions
under which a rock h as been formed. If the rock is igneous, the texture may tell whether intrusive or extrusive; if sedimentary, it tells whether it is an accumulation
or a precipitate: if metamorphic, it tells what changes the
original rock has undergone.
One of the most significant aspects of rock texture depends upon whether the individual particles can be seen
by the unaided eye. Since even the smallest grain becomes
visible when you examine it closely, this statement really
reters to whether the separate minerals are large enough
to be recognized by name, as in typical granite, without
having to be magnified with a lens.
Some igneous rocks contain holes left by the escape of
gas from the lava while it cooled. Pumice is an ideal example. This sort of porous texture or structure can be
useful in recognizing certain extrusive rocks.
The structure of many rocks is layered or banded. This
may mean successive deposits of sediment, each laid on
top of the older ones, as in sandstone. Or it may show repeated flows of lava, or the streaked effect that cooling
magma sometimes gives. Or it may be proof of metamorphism, which squeezes and stretches the minerals until
they take on a ribboned appearance, as in gneiss.
Until you are able to recogni7:e the origin of an un·
known rock- this skill will come with practice in collect161
ing rocks and reading about them-you can at least determine whether it can be split into layers with a prospector's pick. This experiment classifies the rock as either
cleavable or not cleavable.
ROCK KEY NO. 2
COLOR
Color is even less reliable a guide to the identity of rocks
than it is for minerals, for the obvious reason that most
rocks are composed of several different minerals, each of
which may have a quite different color. When the size of
the individual mineral grains is small, however, a rock
may present an over-all color easy to describe and compare.
Just is marble ranges from snow-white to jet-black, so
many other rock.s also occur in a variety of colors. Those
colors, however, which are fairly characteristic of certain
rocks are used in the outline and descriptive section for
the purpose of helping you to name them as readily as
possible.
Light-colored igneous rocks such as granite tend to be
lighter in weight, that is, have a lower specific gravity,
than dark-colored ones. This is because the iron that so
often makes rocks dark also makes them heavy. In the
outline and descriptive section, therefore, color and
weight are given together. The relationship is not always
dependable, though, bec2use some lightweight specimens
are dark as a result of structural peculiarities; obsidian,
for example, has the chemical composition of granite and
weighs even less, but it is often black because of tiny dustlike specks which absorb the light.
ROCK KEY NO.
3
ACID TEST
The carbonate rocks, such as limestone and marble, will
fizz, or effervesce, when touched with acid, owing to the
evolution of carbon dioxide gas. A good idea is to gouge
the rock with a knife blade, producing a powder which
dissolves more rapidly than the solid rock. Any acid may
be used, but ordinary household vinegar will do.
162
Many. igneous t?cks, especially those from are deposits,
have velDS of calClte or some other carbonate mineral in
them and appear to effervesce vigorously. The action is
confined to certain places and soon ceases, however,
whereas a true carbonate rock will continue to fizz all
ove~ for a long time if enough acid is applied.
Pieces of carbonate minerals and rocks may similarly be
present in a sedimentary rock, such as conglomerate or
sandstone. These should not be confused with the bulk
of the rock, which may scarcely react at all. Even more
deceptive is the fizzing, often violent, of carbonate cement
in a sedimentary rock, particularly sandstone. With such
a rock it is necessary to wait and see whether the grains
themselves are also dissolving or only the cementing
material is being affected.
ROCK KEY NO.
4
MINERAL CONTENT
Rocks can best be named by recognizing their individual
minerals where visible. This method requires a knowledge of the physical properties of minerals, as described in
the previous section of this book. Because the mineral
grains in rocks are generally so much smaller than when
found as separate mineral specimens, a sample collection
of good-sized pieces is worth buying for a few cents each
from an established mineral dealer, so that you will become familiar with them and be able to recognize them
when you meet them again in smaller grains in ordinary
rocks.
Most rocks are named scientifically according to their
mineral content. A simplified classification of this sort is
used in this book as a secondary means of identifying the
coarse-grained igneous rocks, in which the minerals are
large enough to be recognized by anyone familiar with
their appearance.
Identifying the Rocks. Outline of Keys
A. Minerals easily seen with unaided eye Page 165
1. One kind of mineral set in finer-grained background Page 165
2. Minerals in layers Page 166
8. Minerals mutually intergrown Page 168
a. Light color; light weight Page .68
b. Dark color; heavy weight Page 172
B. Minerals not easily seen with unaided eye Page 174
1. Metallic appearance
Page 174
2. Coallike appearance Page 175
3. Glassy appearance Page 176
a. Solid body Page 176
b. Porous body Page 177
4. Stony appearance Page 178
a. Can be split into layers Page 178
b. Cannot be split into layers Page 181
1. Fizzes in acid
Page 181
2. Does nOl fizz in acid Page 18S
PORPHYRY
KEYS : Minerals easily seen with unaided eye. One kind of
mineral set in finer-grained background.
Color: Variable. Acid Test: Does not fizz in acid.
Mineral Content: Variable .
The term porphyry is used for igneous rocks that exhibit
individual grains or crystals of a mineral set "like plums
in a pudding" against a general background of finer tex·
ture. The background is called the groundmass, and the
larger grains- the "plums"-are called phenocrysts. As
long as there is a distinct difference in size between the
two parts of the pattern, either can be of any degree of
coarseness. For instance, the groundmass may consist of
grains so tiny that it appears to have a uniform surface,
and then the phenocrysts need only be large enough to
be separately visible. Or the groundmass may be as coarse
as granite, in which case the phenocrysts will have to be
of substantial size, each perhaps several inches or more
acros~. Porphyry is thus a descriptive term, incl uding
rocks of quite different composition. Thus, there are
granite porphyry, syenite porphyry, monzonite porphyry,
gabbro porphyry, felsi te porphyry, and basalt porphyry.
Miners and prospectors in the American Wes t, however,
commonly use the term for almost any fine-grained rock
fou nd in dikes associated with ore deposits.
GNEISS
KEYS: Minerals easily seen with unaided eye. Minerals in
layers.
Color: Variable. Acid Test: Does not fiu in acid.
Mineral Content: Feldspar, quartz, biotite, hornblende.
Coarse
bandjnq_~~~~:::;;iiSE::i~_~
Pronounced "nice," this rock is coarsely banded, the different layers of minerals being roughly parallel in a wide
curving pattern. Dark and light bands occur in succession
across the specimen, and each band may consist of several
different layers. The minerals are predominantly feldspar
and quartz, but biotite mica and hornblende are fairly
abundant as well. Gneiss is a metamorphic rock created
when either an igneous or a sedimentary rock is subjected
to heat and enormous pressure. This causes it "to be
squeezed and stretched, pulling the minerals into stringers which occasionally narrow and widen along their
length. An interesting variety is augen-gneiss, in which
oval fragments of feldspar look like eyes peering out of
the rock. As the bands become thinner, gneiss grades into
another Tock, called schist. Although it can be almost
any age, gneiss is especially frequent in the older rocks,
which have had full occasion to be buried deeply under
thick layers of sediments and to be involved in the mountain-making processes. Fine examples of gneiss occur in
the Highlands of Scotland, in Scandinavia, and in eastern
Canada.
166
CONGLOMERATE
KEYS: Minerals easily seen with unaided eye. Minerals in
layers.
Color: Variable. Acid Test: Does not fizz in acid.
Mineral Content: Quartz, feldspar, variable.
The individual fragments in the sedimentary rock called
conglomerate may be the size of gravel, or as large as
boulders, or a mixture of various coarse sizes larger than
sand. A filling of sand generally occupies the spaces between them. The particles may consist of a single mineral, usually quartz or feldspar, or they may be a mixture
of many minerals or pieces of rock. The cementing material between the grains, though mostly hardened sand,
may also be clay, calcite, iron oxide, or silica. Puddingstone is a conglomerate which shows a distinct contrast
between its large pebbles and the dense matrix in which
they are enclosed. When the fragments have sbarp corners, not well rounded by stream action, the rock is
known as a breccia. Conglomerates otherwise are deposited mostly by water, though some are the result of glacial
action. The most extensive example of this rock in the
United States, formerly called the Great Conglomerate,
lies underneath the coal beds in Pennsylvania and adjacent states, Another Great Conglomerate, in northern
Michigan and Wisconsin, is 2,200 feet thick. Ancient conglomerates are widespread in Canada.
GRANITE
KEYS: Minerals easily seen with unaided eye. Minerals mutually intergrown. Light color; light weight.
Acid Test: Does not fizz in ocid.
Mineral Content: Feldspor, quortz, biotite, hornblende.
The igneous rock that consists of potash teldspar and
quartz in readily visible grains of about equal size is
called granite. Other minerals, especiall y plagioclase
feldspar, biotite mica, and hornblende, may be present;
but potash feldspar and quartz are essential, and they
predominate. Granite should typically be a light-colored
rock. It is true that the color is normally white or light
gray, but when the feldspar is darker because of its structure, the rock is correspondingly dark. The well-known
Quincy granite from Massachusetts is dark-gray, and even
red granite, as it occurs so extensively in Minnesota and
Scotland, is not uncommon. Though it may appear dark,
granite is less heavy than the true dark rocks-the socalled "basic" rocks, such as gabbro-which are dark because of their prominent iron content. The cores of many
of the world's great mountain ranges are composed of
granite, and elsewhere it is deeply hidden. Granite is one
of the major building and monument stones. A strikingly
C\lrious variety, called orbicular granite, is pock-marked
with large knobs of the granite minerals.
168
PEGMATITE
KEYS: Minerals easily seen with unaided eye. Minerals mutually intergrown. Light color; light weight.
Acid Test: Does not fizz in acid .
Mineral Content: Feldspa r, q uartz, muscovite.
Extremely coarse igneous rock. is called pegmatite, and
the bodies themselves are spoken of as pegmatites. The
minerals in the majority of specimens are perhaps twice
as large as those in granite, but frequently they grow to
giganlic sizes: the 47·[00t spodumene in the Black Hills
is the largest in America. Superlative crystals, unexcelled
for perfection and bea uty, as well as for size, are a feature of this kind of rock. The usual minerals in pegmatite
are the same as those in granite-quartz and potash feldspar, mostly microcline. The mica is chiefly muscovite. A
distinctive variety of pegmatite is graphic granite, so
named because the grains of quartz and feldspar are intergrown in an angular fashion, so as to resemble ancient
writing. Noted for their rare minerals as much as for their
coarse texture, pegmatites are the home of fine gems and
other minerals that are not found anywhere else. Rose
quartz, smoky quartz, and moonstone are gems for which
pegmatite is the main original source. Although the
known masses of pegmatite may be of almost any shape,
they are so commonly tabu lar that they are often referred
t o as pegmatite dikes.
SYENITE
KEYS: Minerals easilr seen with unaided eye. Minerals mutually intergrown. ight color; light weight.
Acid Test: Does not fizz in acid.
Mineral Content: Feldspar. biotite. hornblende.
de
A rock very similar to granite in appearance but having
little or no quartz is called syenite. The chief mineral is
potash feldspar; in addition there is usually some plagioclase feldspar and a small amount of the dark minerals
such as hornblende and biotite mica. The feldspar in
syenite has a tendency to assume a rectangular shape. The
name came from the old locality of Syene (now Aswan)
in Egypt, which furnished a stone extensively used for
obelisks in the times of the Pharaohs. The largest
amount of syenite in the United States appears in the
Adirondack Mountains, while other bodies are found in
the White Mountains and the Rockies, and at the edge
of the Ozark Mountains near Little Rock, Ark. In comparison with granite. however, syenite is relatively uncommon. Syenite serves the same commercial uses as
granite, though it is more resistant to fire because of the
absence of quartz. Larvikite is an exceptionally handsome syenite from Larvik, Norway, the feldspar of
which shows a beautiful blue opalescence; this stone decorates the front of office buildings in New York and other
cities in America and Europe.
170
MONZONITE
KEYS: Minerals easily seen with unaided eye. Minerals
mutually intergrown. light color; light weight.
Acid Test: Does not fizz in acid.
Mineral Content: Feldspar, biotite, hornblende, pyroxene.
Closely resembling granite and syenite in the size and
pattern of the minerals of which it is composed, monzonite is regarded as an intermediate igneous rock. When
molten, it has less silica than molten granite; whatever
amount was present has been taken up by the various
silicate minerals, leaving little or none for the forming
of quartz, which is pure silica. Having more iron than
granite has, it contains more of the dark minerals, but not
so many as gabbro, which has a still·higher content of
iron. Hence monzonite stands between granite and gabbro in classification. Its essential minerals are about equal
proportions of both kinds of feldspar-plagioclase and
potash feldspar; the dark minerals present are biotite
mica, hornblende, and augite. Since the two kinds of
feldspar-usually so alike in appearance-are somewhat
different in color when they occur together, monzonite is
rather easily recognized by this fact of one being gray
and the other white, or one white and the other pink.
This is an important rock in connection with the metalbearing mineral deposits in Colorado and elsewhere in
western North America.
GABBRO
KEYS: Minerals easily seen with unaided eye. Minerals mutually intergrown. Dark c:olor; heavy weight.
Add Test: Does not fizz in ocid.
Mineral Content: Feldspor, pyroxene, hornblende, olivine.
Feldspar
An igneous rock which is dark and heavy and which con·
sists of plagioclase feldspar and other mineral grains
large enough to be visible and recognized is called gabbro. The minerals are typically black, dark-gray, or dark·
green in color, being chiefly plagioclase feldspar (mainly
labradorite) and pyroxene (commonly augite), with
some hornblende and olivine. When gabbro is composed
almost exclusively of labradorite, the rock is called
anorthosite, of which large amounts occur in Wyoming,
eastern Canada, and Scandinavia. Another kind of gabbro, called norite, encloses huge deposits of nickel ore,
especially at Sudbury, Onl., and chromium and platinum
ore, as in South Africa. Gabbro is a deep-seated rock,
formed at high temperatures. It is a familiar sight in the
vicinity of Baltimore, Md.; at Duluth, Minn.; and elsewhere around Lake Superior; and in the Adirondack
Mountains. Less popular than granite on account of its
somber appearance, gabbro is easier to work; in Sweden
it is favored for monuments and sculpture. When it decomposes, it yields a reddish soil because of the high con·
tent of iron in the dark minerals.
17~
PERIDOTITE
KEYS: Minerals easily seen with unaided eye. Minerals mutually intergrown. Dark color; heavy weight.
Acid Test: Does not fizz in odd.
Mineral Cont ent: Olivine, pyroxene, hornblende.
A dark rock resembling gabbro but heavier and containing little or no plagioclase feldspar is called peridotite.
I t has a comparatively high content of iron, and the color
is dark green or black. The dominant mineral is olivine.
and it is the French word (peridot) for this mineral that
has given us this name. Pyroxene (mostly augite) and
hornblende are less in quantity. Peridotite has formed
under conditions of maximum temperatures and often
considerable pressure. I t sometimes is a valuable source
of metals notably nickel, chromium, and platinum, and
in Rhode Island it contains a good deal of magnetite. The
most significant mineral ot any to be found in it is diamond. The diamond-bearing variety-the only proved
mother rock ot this gem-was named kimberlite after the
South African city which is the center of the industry.
T he same occurrence of diamond in peridotite is known
in Pike County, Ark., and in India. Other bodies of
peridotite, but without diamonds, are distributed
throughout the world. Tremendous volumes are present
in New Caledonia and Cuba. Peridotite weathers readily
to serpentine, with which asbestos may be associated.
173
METEORITE
KEYS: Minerals not easily seen with unaided eye. Metallic
appearance.
Color: Dark. Acid Test: Does not fizz in ocid.
Mineral Content: Iron, olivine.
Black or brown crust
Although so rare that the average mineral collector is unlikely ever to hnd one, meteorites are of such vital scientific importance that the chance of your coming across
a specimen and failing to recognize it should be reduced
as much as possible. Only about 1,550 different meteorite
"falls" have been recorded to date, though some falls are
represented by many separate fragments. Each fall is different from the rest, and an expert can tell trom which
locality any given specimen has come. The classification
of meteorites embraces three major kinds. Siderites (not
to be confused with the mineral siderite), or so-called
iron meteorites, are metallic, consisting of an alloy of
iron, nickel, and cobalt. Aerolites, or so-called stony
meteorites, resemble many ordinary heavy rocks; consisting mainly of olivine and pyroxene, they always reveal at
least traces of metal. Those of an intermediate type are
siderolites or ironstones, an example of which are the
pallasites, showing nuggets of olivine set in a handsome
meshwork of metal. The brown or black iron oxide coatings and the depressions or "thumbprints" are characteristics of meteorites.
174
COAL
KEYS: Minerals not easily seen with unaided eye. Coal.
like appearance.
Color: Black. Acid Test: Does not fizz in acid.
Mineral Content: None.
Banded
Coal is regarded as a sedimentary rock because it is found
in layers or beds. All coal once existed as growing plants
which died, became partly decayed, and then were preserved by burial. The original plant life was of an amazing variety, over 3,000 different species having been
identified from the age of greatest coal making. Appropriately called the Carboniferous Period, this was a time
of lush vegetation, when ferns grew the size of to day's
trees and rushes were 90 feet tall. The first stage in the
forming of coal is called peat. As further burial continues,
the gases and water are forced out and the material left
behind is thereby enriched in carbon. Peat thus turns to
lignite or brown coal, whid. gives way to bituminous or
soft coal, and eventually to anthracite or hard coal. As
these changes proceed, the coal becomes brighter and
harder, and it breaks more regularly. If coal is squeezed
by pressure from the sides during the building of a mountain range, the ultimate product may be the mineral
graphite. The thickest seams of coal are situated in Victoria, Australia, and the most valuable are in northeastern Pennsylvania.
OBSIDIAN
KEYS: Minerals not easily seen with unaided eye. Glassy
appearance. Solid body.
Color: Black. Acid Test: Does not fizz in acid.
Mineral Content: Glass.
When lava flows onto the surface of the ground and cools
so quickly that separate minerals do not have time to
form, the resulting natural glass is known as obsidian. Its
chemical composition is such that it would have become a
normal granite if it had solidified very slowly at depth.
The usual jet·black color is due to the presence of tiny
specks of magnetite scattered like dust so that they absorb the light. Inside, also, may be seen the beginnings
of crystals which failed to grow further, and often appear
in flowerlike spots. Owing to its tendency to break with
sharp edges, this natural glass was a boon to the people
of primitive races who used it for all sorts of implements
and weapons. Obsidian Cliff in Yellowstone National
Park is a noted occurrence, and Mono Lake, Calif., is
another. Distinctive obsidian from certain islands in the
Mediterranean is mottled in red and black and appears
somewhat pitchy. True pitchstone, however, is natural
glass with a higher water content than obsidian and a
pitchy luster. Ultimately obsidian seems to crystallize, so
that all natural glass dates from relatively modern geologic time.
176
PUMICE
KEYS: Minerals not easily seen with unaided eye. Glassy
appearanc:e. Porous body.
Color: Gr~y. Acid Test: Does not fizz in acid.
Mineral Content: Glass.
Expanded by the explosion of steam as it escapes from
a volcano and puffed up into a froth of glass, pumice is a
foam y m ass of silky glass shards. These shards may be intermixed or else drawn out in parallel strands. Its porous
nature and the many isolated cells of air enable pumice
to float on water for a long while, pieces of it drifting
ashore on almost every seacoast before they become
waterlogged. The chemical composition of pumice is like
that of obsidian or granite, except as the peculiar conditions under which it was formed make a difference. Although most pumice is light-colored, some of it is brownish or occasionally red. The sharp cutting edges of the
bits of glass make pumice a serviceable abrasive. used in
scouring-soap and as a dental polish. The Lipari Islands,
between Sicily and the mainland of Italy, have long been
the leading source of commercial pumice. New Mexico,
California, and Oregon produce the most American
pumice. A thick bed of California pumice is mined and
sliced for use as an inSUlating material in refrigerators.
Smaller pieces are mixed with cement and plaster to give
these construction materials a lighter weigh t.
SHALE
v
KEYS: Minerals not easily seen with unaided eye. Stony
appearance. Can be split into layers.
Color: Gray, black. Acid Test: Does not fizz in acid.
Mineral Content: Clay.
-Thin
layers
The sedimentary rock called shale consists of various clay
minerals, which have ceased to be plastic and have accumulated into thin beds. The flaky grains are too tiny
to be recognized at sight, but tests show them to be
kaolinite and the related aluminum silicate minerals collectively known as clay. Small flakes of white mica, bits
of quartz sand, and cementing limestone are commonly
present, and almost any mineral can be incorporated into
a shale. Lumpy inclusions called concretions are sometimes large. Shale is a soft and easily eroded rock, and it
can be split with little effort. Some black shale is rich in
carbon; other shale is gray or almost any color. Fuel has
been produced from oil shale in Scotland for a century;
the colossal reserves in western Colorado and eastern
Utah are said to be the largest mineral deposit in the
world except the ocean. Its weakness prevents the use of
shale as a building stone, but this rock, abundant
throughout the globe, has value as a mixing substance in
the manufacture of cement. As shale increases in coarseness by the addition of sand, it grades into siltstone.
178
SCHIST
KEYS: Minerals not easily seen with unaided eye. Stony
appearance. Can be split into layers.
Color: Vorioble. Acid Test: Does not fizz in ocid.
Mineral Content: Yorioble.
Schist is a closely layered metamorphic rock. Its name
comes from the Greek word meaning "to divide," because of the ease with which it can be split between the
layers. These layers are narrower than those of gneiss, but
the two kinds ot rock grade into each other, and no definite line can be drawn between them. As a rule, adjacent
layers in a schist consist of the same minerals, and so
schist is much more uniform in appearance and composi.
tion than gneiss. To this fact it owes much of its ability
to separate by splitting. Unlike gneiss, schist is not so
likely to contain feldspar as a significant mineral. Mica,
instead, is extremely common; mica schist, composed of
quartz associated with abundant flakes of black biotite or
silvery muscovite, is the most widespread variety of schist
and separates the most readily. Hornblende schist is an·
other important rock, and there are other kinds of
schist marked by red garnet, green chlorite, shiny-gray
graphite, or white talc. These often make colorful speci·
mens. As the layers of schist become still narrower, the
rock grades into phyllite and then into slate.
SLATE
KEYS: Minerals not easily seen with unaided eye. Stony appearance. C an be split into layers.
Color: Groy. Acid Test: Does not fizz in ocid.
Mineral Content: Cloy.
Intense pressure acting on beds of shale changes them to
slate, a metamorphic rock. As a result, a smooth cleavage
is developed at an angle to the original bedding and
straight across the direction of pressur~. The individual
grains, like those in shale, are too small to be visibly
recognized without magnification. Although the normal
color of slate is dark gray, inclining toward black, varieties are known which are red, green, purple, or brown.
Some slate contains conspicuous crystals of pyrite, wellshaped and attractive against their dark background.
Another kind of slate with inclusions is knotted slate,
having coarse crystals of silicate minerals scattered
through it; this slate indicates the nearness of an igneous
rock and a possible ore deposit. As might be expected,
slate occurs in mountainous regions where the required
pressure was developed in past ages. Commercial production in the United States, for blackboards and roofing
purposes, is carried on mostly in Pennsylvania and Vermont. Broad, thin sheets of extraordinary size are quarried at Pen Argyl, Pa. Otherwise useful deposits may be
too far from adequate markets.
180
LIMESTONE
KEYS: Minerals not easily seen with unaided eye. Sfony
appearanee. Cannot be split into layers. Fines in acid.
Color: White. Mineral Content: Colcite, dolomite.
Limestone is a sedimentary rock composed solely of one
mineral, calcite. It is generally white or gray. except
when impurities cause a darkening. Limestone can be
formed through the life processes of a wide range of
organisms. from one-celled plants, such as algae. to specialized animals. such as corals. These extract calcium
carbonate from the water and use it to build their skeletons and shells. Limestone also can e deposited inorganically. by strictly chemical means. There are many
different kinds of limestone. A well-known one is chalk,
which seems to be a fine powder until seen under a microscope, when it proves to be made up of the tiny plates of
a\~e and the minute shells of low1'l forms of animals.
Coquina is limestone composed of an accumulation of.
loosely packed shells. cemented together firmly enough
so that it is used in Florida as a building stone. Travertine is 'porous limestone deposited by hot springs. Socalled Mexican onyx. familiar in pen stands and clock
cases. is limestone marked by swirling patterns in attractive colors. Most cave stalactites and stalagmites are built
of limestone.
MARBLE
KEYS: Minerals not easily seen with unaided eye. Stony
appearance. Cannot be split into layers. Fines in acid.
Color: White, gray. Mineral Content: Calcite.
-
Rhomb·lc cleavQge.
When either limestone or dolomite is drastically changed
by heat, pressure, and water, the new metamorphic rock
is termed marble. The agents that produce this transformation enable the grains of calcite or dolomi te to
grow larger and to give the surface a livelier sparkle than
before. Any impurities that were present in the original
rock or that were introduced during the change tend to
be segregated into knots or spread out in wavy streaks,
producing the varicolored "marbling" that is so appealing a feature of ornamental marble. North Africa produces superb examples of such stone. Snow-white marble,
such as the famous stone from Carrara, Italy, is favored
for statuary purposes. Similar Pentelic marble from
Greece was sculptured into the priceless creations that
are so enduring a heritage of Hellenic civilization. The
Yule marble from Colorado, of which a single block
weighing 56 tons was required for the Tomb of the Unknown Soldier, is equal to any from the Medi~erranean
region. Architectural marble is characterized by its uniform color. Vermont, Tennessee, and Georgia are states
quarrying important amounts of marble.
182
SANDSTONE
V'
KEYS: Minerals not easily seen with unaided eye. Stony
appearance. Cannot be split into layers. Does not fizz in
acid.
Color: White, brown. Mineral Content: Quartz.
Banded
An accumulation of grains of quartz sand, cemented tog:.ther more or less firmly to become rock, is called sandstone. As sandstone becomes coarse, it grades into conglomerate; as it becomes finer, it grades into siltstone.
Some sandstone, such as the Potsdam sandstone of Wisconsin, is remarkably pure, consisting of little except
quartz. Other specimens contain feldspar in increasing
proportions, flakes of mica, and small grains of heavy
minerals typical of placer deposits. The color of sandstone depends largely on the nature of the cement; iron
oxide gives the red, yellow, and brown shades. A limy cement will fizz in acid but not the mineral grains. The
shape of the individual particles likewise varies from perfectly round to sharply angular. Sandstone that splits
readily into even slabs is known as flagstone. Brownstone.
once the most fashionable building stone in Philadelphia,
New York. and other Northeastern cities. is a sand.;tone
containing considerable reddish feldspar. The most extraordinary kind of sandstone. called itacolumite, is actually
flexible; a slab of it from North Carolina or Brazil will
bend under its own weight.
QUARTZITE ,./
KEYS: Minerals not easily seen with unaided eye. Stony
appearance. Cannot be split into layers. Does not fizz in
acid.
Color: White. Mineral Content: Ou~rtz.
When subjected to pressure or heat, sandstone is changed
to the metamorphic rock know as quartzite. This name,
which indicates that quartz is practically the sole mineraI present, is also applied by many geologists to ordinary sandstone which has merely been cemented together
more tightly than usual. A true quartzite, however- the
toughest of all Tocks-will break across the sand grains instead of having to break around them, because the former
cementing material in the original sandstone has become
as hard as the grains themselves_ During the processes of
change, moreover, some of the previous cement is likely
to be recrystallized into small quantities of a number of
new minerals, such as garnet, epidote, graphite, and
muscovite mica. T hese may give color to an otherwise
white rock. Quartzite used industrially as a heat-resisting
material is called ganister. Burrstone is a porous variety
of quartzite used for millstones. The Quartzite Range in
British Columbia is named from its exposures of very old
quartzite. The Baraboo quartzite is a handsomely crystalline rock, often deep-red in color, well exposed in
Wisconsin.
184
FELSITE
KEYS : Minerals not easily seen with unaided eye. Stony
appearance. Cannot be split into layers. Does not fin in
acid.
Color: W hite, gray. Mineral Content: Feldspa r.
May be
porous---+.o:
Usually
banded __________~~~
Light-colored igneous rocks that are so fine-grained that
the constituent minerals can scarcely be recognized without a microscope are grouped together under the name
felsite. They may be of any color but are light in tone,
as contrasted with basalt, which is dark. The difference is
basically due to the kinds of minerals that are present,
and this is impossible to determine with the unaided eye.
With a hand lens, however, perhaps the translucent edges
of the feldspar, which is the chief mineral in felsite, can
be observed even in some of the darker-looking specimens.
Felsite originates mainly in lava flows and is abundant on
a world-wide scale. One of the most important types is
rhyolite, which makes up a large part of Yellowstone National Park. Phonolite contains the gold ores at Cripple
Creek, Colo. It emits a ringing sound when struck; hence
its name. Trachyte is promment along the Rhine River,
in central France, and in the Black Hills of South Dakota.
Andesi te is represented by a series of high volcanic cones
stretching £rom Mou nt Rainier, Wash., through Mexico
and Central America, all the way to the far tip of the
Andes.
BASALT
KEYS: Minerals not easily seen with unaided eye. Stony
appearance. Cannot be split into layers. Does not fin in
acid.
Color: Black. Mineral Content: Feldspar, pyroxene, olivine.
Usually has
green inclusions
Usually
porous
A heavy dark rock with a fine-grained texture, basalt is
the most abundant of the rocks which have cooled from
sheets or flows of lava. The gas escaping tram the molten
matter as it rises to the surface of the earth leaves oval
cavities, which later are often lined or completely filled
with minerals. The chief mineral that is readily visible
against the dark background is olivine, which usually
appears as green particles the size of buckshot. Of all
the various kinds of rock, basalt is the one most likely to
be divided by columnar jointing, which produces long upright pillars. The scenic Giant's Causeway on the north
coast of Ireland is perhaps the most impressive example.
Basalt occurs in small bodies as well, but the largest ones
are of incredible extent. The Hawaiian Islands are built
almost entirely of basalt, which erupted from numerous
vents. The Columbia River region in the Northwestern
section of the United States was blanketed by basalt, to
create much of its present landscape. The Deccan region
of India contains nearly one-quarter million square
miles of basalt, which was piled as much as 10,000 feet
thick.
186
Magazines for the Collector
The following magazines are national publications devoted to mineral and rock collecting and related hobbies.
In addition there are various smaller but good periodicals
of regional circulation and numerous specialized professional journals which may be consulted in public or college libraries.
Gems and Minerals, edited by Don MacLachlan. has short illustrated articles on the various phases of mineral collecting. It is pub·
lished monthly at Mentone, Calif.; the subscription price is $3.00
per year.
The Mineralogist, edited since 1935 by Dr. H. C. Dake. deals interesting)y with all aspects of the mineral hobby. It is published
bimonthly (except in the summer) at 329 S.E. 32d Avenue, Portland
15, Ore.; the su bscription price is $2 .00 per year.
Rocks and Minerals, edited since 1926 by Peter Zodac. covers in
popular language the entire field of minerals and rocks. It is published bimonthly at Box 29, Peekskill, N. Y.; the subscription price
is $3.00 per year.
Earth Science, edited by Dr. Ben H. Wilson. includes worthwhile
articles on minerals, rocks. and popular geology. It is published
bimonthly at Box 1357. Chicago 90, Ill.; the subscription price is
$2.00 per year.
Th e Lapidary Journal, edited since 1947 by Lelande Quick.
specializes in the collecting and cutting of gems and ornamental
stones. It is published bimonthly at Del Mar. Calif.; the subscription price is $3.00 per year.
The Desert Magazine, edited since 1938 by Randall Henderson.
features articles on mineral localities in Southwestern United States.
illustrated with helpful maps. It is published monthly at Palm
Desert. Calif.; the subscription prke is $3.50 per year.
Books fo,. the Collector
The following books are especially recommended for further reading and reference. The classifications and brief
descriptive notes are intended to aid the purchaser.
INTRODUCTORY
Getting Acquainted with Minerals by George L. English. revised
by David E. Jensen. published by McGraw·Hill Book Company. Inc ..
New York (2d ed .• 1958). An attractive book to stimulate a beginner.
TEXTBOOK
Minerals and How to Study Them by Edward Salisbury Dana.
revised by Cornelius S. Hurlbut. Jr .• published by John Wiley Be
Sons. Inc.• New York (3d ed .• 1949). Well illustrated.
REFERENCE
roor Questions Answered About the Mineral Kingdom by Richard
M. Pearl. published by Dodd. Mead 8< Company. New York (1959).
Deals with various aspects of minerals and rocks.
GEMS
Popular Gemology by Richard M. Pearl. published by Sage Books.
Denver (2d ed .• 1958). A scientifically accurate. finely illustrated
book on gems. clearly written in layman's language.
PROSPECTING
Successful Prospecting and Mineral Collecting by Richard M.
Pearl. published by New American Library of World Literature.
Inc .• New York (1959). llIustrated book on both phases of mineral
collecting.
LOCAUTIES
Gem Hunter's Guide by Russell P . MacFall. published by Science
and Mechanics Publishing Company. Chicago (2d ed .• 1958). Brief
mention of many collecting localities throughout North America.
Colorado Cem Trails and Minera l Guide by Richard M. Pearl.
published by Sage Books. Denver (1958). A detailed guide with many
maps.
N umerous other regional locality guides are available.
188
Index
Achroite. 154
Acid test. 161
Actinolite. 110
Aerolite. 174
Agate. 15t
Alabaster. 97
Albite. 125
Almandite. 158
Amazonstone. 115
Ameth yst. 151
Amphibole. 110
Analcime or
analcite. 144
Anatase. 88
AndaJusite, 157
Andesite. 185
Andradite. 158
Anglesite, 107
Anh ydri te, 105
A nna bergi te, 66
Anor thosi te, 171
Anthoph yllite. 110
Antigorite, 1311
Apatite. 136
Apoph ylJite. 121
Aquamarine. 159
Aragonite. 1411
Arfvedsonite, 110
Arsenopyrite. 8t
Asbestos. 1113
Asparagus stone. 1116
Augite. log
Aztec money, 143
Azurite. 86
Baguette cut. 11
Barite. 101
Barite rose. !l2. 101
Basalt. 186
Basic rocks. 168
Bauxite. 150
Beidellite. 131
Beryl. 159
Biotite. 95
Black sand. 24
Blackjack. 93
Blende.93
Blistered copper
ore. 65
Bloodstone. 152
Blue john. lilt
Boehmite. 130
Borax. 135
Bornite. 68
Breccia. 167
Brilliant cut. 11
Brimstone. 129
Brookite. 88
Brownstone. 183
Burrstone. 184
Cabinet. mineral. 36
Cabochon cuts. 11
Cairngorm. 151
Calamine. 117. "9
Calcite. 99
Califomite. 147
Carnelian. 152
Carnotite. 92
Cassiterite. 94
Celestite. 102
Cerussite. 107. 134
Chalcedony. 152
Chalcocite. 76
Chalcopyrite. 65
Chalcotrichite. 69
Chalk. 181
Chal ybite. 115
Chemical formulas.
51
Chiastolite. 157
Chlorite. g6
189
Chromite. 85
Chrysocolla. 87
Chrysoprase. 152
Chrysotile. 1!l3
Cinnabar. 89
Cla y. 15. 1!l1
Clay ironslOne. uS
Cleaning specimens.
115
Cleavage. 45
Clinochlore, 96
Coal. 175
Colemanite, 121
Color. 4!1. 162
Concretion, 26. 178
Conglomerate. 167.
18 11
Copper. 611
Copper nickel. 66
Coquina. 181
Corundum, 126
Cotton-ball borax,
121
Covellite. 56
Cryolite. 104
Crystal axes. 12
Crystals. II
growing of. 1I8
Crystal systems. II, 11
Cuprite,6g
Datolite. 14:1
Demantoid. 158
Desert rose. 1It
Diamond. u8
Diaspore. I!l0
Dickite. I!P
Diopside. 109
Dogtooth spar. 99
Dolomite. 100
Dravite, 154
190
Itacolumite, 18S
Geiger counter. 50
Gems. 15
Geodes. 15. 14. 51,64, Jacinth. 156
100
Jade. log. 110
Geysers. 22
Jadeite. log
Jasper. 152
Effervescence. 47. 162 Gibbsite. 130
Glacial ore. 76
Joints. 21. 25. 26. 17
Elasticity. 48
Glaciers. 24
Emerald. 159
Glaucophane. 110
Emerald cut. 11
Kaolin or kaolinite,
Gneiss. 166. 179
Emery, 126
1!1 1
Goethite. 70
Enargite. 72
Kernite. 106
Gold.
62
Endlichite. 1!l2
Kidney ore. 79
Granite. 168
Enstatite. 109
Kimberlite. 175
Graphic granite. 169 Kunzite. log
Epidote. 124
Grapbite.
60.
175
Equipment for
Kupfernickel. 66
Grossularite. 158
collecting. !l5
Kyanite. 111
Groundmass. 165
Extrusive rocks. 19
Gypsite, 97
Labeling specimens.
Gypsum. 97
Faceted cuts. 11
!l5. !l6
Labradorite. 125
Fairy cross. 155
Haarkies. 64
Lapis lazuli. loS
Famatinite. 72
Half-breeds. 65
Laurvikite. 170
Fayalite. 146
Halite. 10!l
Lazurite. loS
Feldspar. 125
Halloysite. 151
LepidOlite. 95
Feldspathoid
Hardness. 42 • 45
Leucite. 150
minerals. 150
Hauynite. 108
Lignite. 175
Felsite. 185
Hematite. 79
Limestone. 181
Ferberite. 81
Hemimorphite. 1 '9
Limonite. 70
Ferrimolybdite. 59
Hexagonal system. III Lodestone. 80
Flagstone. 18!1
Hornblende. 110
Luminescence. 50
Flexibility. 48
Horseflesh ore. 68
Luster. 41
Flint. 15. 152
Hot springs. 2lt
Flos ferri. 145
Huebnerite. 81
Fluorescence. 49
Magma. 19
Hyacinth. 156
Fluorite. 112
Magnesite. 145
Fool's gold, 62. 65. 77 Hypersthene. log
Magnetic pyrite. 67
Forsterite. 146
Magnetism. 49
Fossils. 26. !II
Magnetite. 80
Iceland spar. 99
Fowlerite. 115
Magnifying glass. 54
ldocrase. 147
Fracture. 46
Igneous rocks. 19. lilt Malachite. 85
French chalk. g8
Malleability. 49
Ilmenite. 84
Fumaroles. 21
Marble. 182
Indian dollar. 145
Fusibility. 51
Marcasite. 78
Indicolite. 154
Marquise cut. 11
Industrial minerals
Metallic minerals. 18
and rocks. 15
Gabbro. 171
Metals. 16
Intrusive rocks, 19
Gahnite. 160
Metamorphic rocks.
Iron rose. 79
Galena. 57
Ironstone. '74
27
Ganister. 184
Meteorite. 174
IIometric system. 11
Garnet, 158
Dry-bone ore. 117
Dumps. !p
Dunes. 24
Dunite. 146
Mexican onyx. 181
Peacock ore. 68
Mica. 95
Peat. 175
Microcline. 125
Pegmatite. 18. 16g
Micromounts. 12. IS. Pencil ore. 79
Penninite. 96
S2
Millerite. 64
Pentlandite. 67
Mimetite. IS'
Peridot. 146
Minerals. definitioD
Peridotite. 146. 175
of.
Phenocryst. 165
Mispickel. 82
Phlogopite. 95
Mohs scale of hard- Phonolite. 185
Phosphorescence. 50
ness. 42
Molybdenite. 59
Phyllite. 1'9
Monoclinic system.
Pitchblende. 74
Pitchy copper ore. 87
12
Montmorillonite. 1!1l Placers. 25
Monzonite. 1,1
Plagioclase. 125
Polianite. 61
Moraines. 25
Porphyry. 165
Morganite. 159
Moss agate. 61
Porphyry copper. 76
Powellite. 141
Mullite. III. 157
Prehnite. 149
Muscovite. 95
Prochlorite. 96
Museums. 119
Properties of minerals. 41
Nacrite. 151
Proustite. 75
Nailhead spar. 99
Pseudomorphs. 15
Natrolite. 120
Puddingstone. 167
Needle zeolite. 120
Pumice. 177
Nephrite. 110
Purple copper ore. 68
Niccolite. 66
NonmetalJic minerab Pyrargyrite. 711
Pyrite. 77
and rocks. 15
Pyrolusite. 61
Norite. 1,2
Pyromorphite. 157
Noselite. 108
Pyrope. 158
Pyrophyllite. 98
Obsidian. 176
Pyroxene. log
Olivine. 146
pyrrhotite. 67
Onyx. 152
Opal. 155
Quartz. 151
Opalized wood. 155
Quartzite. 164
Ore deposits. 21
I,
Orpiment. 91
Orthoclase. 125
Orthorhombic
system. 12
Pallasite. 174
Parting. 46
Radioactivity. 50
Realgar. go
Rhodochrosite. J 14
Rhodolite. 158
Rhodonite. llS
Rhyolite. 185
Riker mount. 57
Rock crystal. 151
Rock flour. 24
Rock salt. 105
Rocks. definition of.
I,
Rubellite. 154
Ruby. 126
Ruby copper. 6g
Ruby zinc. 95
Rutile. 88
Sagenite. 88
Salt. 15. lOll
Salt domes. 105. 129
Sandstone. 185
Sapphire. 108. 126
Sardo 152
Satin spar. 97
Scheelite. 141
Schist. 179
Sedimentary rocks. 21
Selenite.
Serpentine. IllS
Shale. 178
Siderite. 115. 174
Siderolite. 174
Siltstone. 178. 18S
Silver. 71
Slate. 179. I So
Smithsonite. 117
Soapstone. 98
Sodalite. 108
Soil. 15. 28
Sparable tiD. 94
Spathic iron. 115
Specific gravity. 47
Specimen mount. 57
Specularite. 79
Spessartite. 158
Sphalerite. 95
Sphene. 140
Spinel. 160
Spodumene. log
Stalactite. 14
Staurolite. 155
Steatite. g8
StibDite, 58
9'
19 2
Stilbite. u8
Stratification. 25
Streak. 44
Streak plate. 44. 45
Stream cycle. 29
Striations. 48
Strontianite. 116
Structure. 161
Sulfur. 129
Syenite. 170
Talc. 98
Tarnish. 44
Tennantite. 75
Tetragonal system.
u
Tetrahedrite. 75
Texture. 161
Tiger·s-eye. 151
Tincalconite. 106.
135
Titanite. 140
Toad's-eye tin. 94
Topaz. 127
Tourmaline. 154
Trachyte. 185
Travertine. 181
Tremolite. 110
Triclinic system. 12
Trilobite. 26
Troilite. 67
Troostite. 138
Turkey-fat ore. 117
Turquoise. 148
Ulexite. 121
Ultraviolet lamp. 49
Uraninite. 74
Uvarovite. 158
Vanadinite. 1!!2
Veins. 21
Verd antique. I!!!!
Vesuvianite. 147
Volcanism or volcanoes. 20
Wad. 61
White garnet. 150
White lead ore. 134
Willemite. 13B
Wolframite. BI
Wollastonite. 123
Wood tin. 94
Wulfenite. 139
Zeolite. liB. 120. 144
Zinc blende. 93
Zircon. 156
VANADINITE
VARISCITE
Near Clobe, !\ ri/ona.
Fairlidd. Ulah.
WAVElllTE
WOLFRAMITE
:-\ear H ot Spring~,
Zinnwald , Bohe mia.
;\r).. a/l~.n
WUlFENITE
ZINCITE
$ieTT3 de los Lamenlos,
Chih uahua , Mexico.
Franklin,
~e\\'
Jersey.